This is a tricky situation, because neither of you is objectively more correct than the other. Your sister makes a valid case, but so do you. The decision on whose argument is more correct is fully subjective.
If I were to put this question in front of a large crowd, I would expect there to be support for either side of the argument, which means that we can't tell you which side is definitively correct.
First things first, you have the right to refuse to bring her cigarettes. Nothing I say is going to change that fact.
But I want to explore that decision first. Not to criticize you or invalidate your opinion, but rather to showcase that your sister is likely hearing something different than what you're saying; which is contributing to the issue.
Let me play your advocate and your sister's advocate. From those two viewpoints, we can distill an IPS approach that avoids offending either party.
Playing your advocate.
You have the right to refuse any favor that is asked of you. Full stop. It doesn't matter whether your sister is asking your to bring her cigarettes or her slippers.
However, the freedom to do so does not mean that you're immune to the consequence of doing so. If your sister find your refusal unjustified, she will be upset with you, even if she can't force you to actually perform the favor.
But how to phrase your refusal?
I strongly disagree agree with smoking and feel like I would be encouraging/condoning it by bringing her cigarettes and I have therefore said I won’t bring her any cigerettes.
If you used a phrasing similar to this, I can understand why your sister essentially ignores your explanation and repeats the question (Note: I can see where she's coming from. I'm not saying that she's objectively correct).
Based on your phrasing, you've implied that you refuse to bring her cigarettes because you want to make her stop smoking. In your question, there is a general underlying tone to most of what you said that implies this same intent; you're trying to direct your sister's behavior.
Importantly: You've already explicitly denied this in a later comment you made, where you confirm that you're not trying to get her to stop. But sadly, that message gets lost in translation, because your initial phrasing implies personal motivations, even if that's not the case.
The source of conflict often derives from making implications, regardless of whether you wanted to make the implication or believe that it's correct.
From this point on, we're focusing on what your sister hears, not on what you intended to say in the first place.
Your sister may not consider this a valid justification. Legally speaking, it isn't (if she's an adult, you have no standing to make her life decisions for her). Morally speaking, it's subjective (some people will agree with your sister, some people will agree with you).
So instead, stick to the undeniable facts:
I feel uncomfortable bringing you cigarettes. You know my opinion on smoking, and bringing you cigarettes would make me feel like I'm going against my own moral code. I would rather not do so.
Everything you say here is undeniably true. Even though your motivation is subjective, describing that this is your motivation is objectively correct.
However, I think it would be interesting for you to consider how unwilling you are to enable her smoking. Forget the hospital for a second:
- Would you be okay asking someone (e.g. another sibling) to bring her cigarettes?
- Would you drive her to a shop if she needs cigarettes?
- Would you tell her where to find a shop where she can buy cigarettes?
- Would you lend her money, if you knew she was going to buy cigarettes with it? (assume she is guaranteed to pay you back promptly)
- Would you accompany her if she drives herself to a shop to get cigarettes? (assuming you would otherwise still have spent time with your sister)
I think you can see that this becomes a slippery slope. There is a point where your refusal will generally be considered petty (by your sister), which will cause friction between you (thus precluding a good IPS approach).
We can argue about where that exact point is, but that's irrelevant. Your opinion is different from your sister's, which is different from mine. There is no objectively correct answer here.
However, if you are in fact comfortable with some of the alternatives (you can of course think of others if you want), I would include them as a suggestion:
I would feel uncomfortable bringing you cigarettes. You know my opinion on smoking, and bringing you cigarettes would make me feel like I'm going against my own moral code. I would rather not do so.
If you want, I can ask [sibling] to bring you some when they visit you.
This lowers the chance for your sister to get offended or consider your refusal unjustified. Instead of abjectly refusing, you are instead simply trying to avoid a personal issue, and have suggested a viable alternative that does not make you feel (as) uncomfortable.
Playing your sister's advocate.
How can I get her to respect my beliefs and stop asking me to help her smoke?
Note that your sister can ask the same of you, getting you to respect her right to smoke if she chooses to, regardless of your opinion on the matter.
Changing the example, would you bring your sister her favorite cookies if she asked you to?
While it may be morally justified for you to refuse doing so if your sister is e.g. in hospital for obesity-related problems; there's no real moral justification for refusing to do so if your sister is in good health (i.e. the cookies are not unusually detrimental to her in her current state).
This is the core of your sister's argument. She is asking for your assistance. The only reason she's asking for help is because she currently has issues doing it herself (due to being confined to the hospital). She is not asking for your permission, approval or advice.
When you refuse to bring your sister cigarettes, what she hears is that you're refusing to help her. You refused her request, but (to her) it was merely a request for help, not to make her life decisions for her.
There are valid reasons for you to refuse bringing her cigarettes, but from your sister's point of view, they are generally limited to not being able to visit or bring anything at all (regardless of the cigarettes themselves).
What I'm trying to get at, is that there is a meaningful difference between helping someone, and helping someone do something.
Let me change the example, so that you're impartial to the scenario:
A man is lying on the ground. He is wounded. I rush over to him, I treat his wounds and nurse him back to full health. Upon recovery, the man continues what he was doing before he got injured: destroying the rainforest.
If I had not helped this man, he would not have been able to destroy more of the rainforest.
Even though I helped the man, and he could not have continued destroying the rainforest without me healing him, I did not help the man to destroy the rainforest.
What the man chose to do after he was healed, was his decision, not mine. Healing the man does not make me responsible for all his future actions.
The same is true for your sister. She is asking for your help, but she's not asking you to kill her by making her smoke. Even after you bring the cigarettes, you're not forcing her to smoke them. She is willingly smoking cigarettes, and it is her right to choose to do so.
Bringing your sister cigarettes does not make you responsible for her voluntarily smoking the cigarettes.
There is one fringe exception: if your sister has already indicated she wanted to stop smoking, and is clearly caving to the withdrawal. But you never mentioned that your sister has wanted to stop smoking, so this is irrelevant for the current situation.
So, what's the conclusion here?
First and foremost, you need to decide where your priorities lie. If there is no middle ground to be found, would you rather bring cigarettes to your sister, or will you keep refusing to do so (and risk having her be upset with you)?
If you will keep refusing to do so; then you have your answer. Phrase your refusal kindly (as per the example above), but since you have definitively made up your mind, there's no further discussion to be had.
All you can do is explain your reasoning to your sister in order to lower the chance (or severity) of her being upset with you, but that's not a guarantee.
It's possible that offending her is unavoidable. That depends on your sister, and I don't know her.
However, if you would rather bring her cigarettes than have a falling out with her, or are at least open to compromise (meeting her halfway), then there is more to discuss. The only fair way for this to work, is if both of you are comfortable with the compromise.
In this case, start off the same way: explain to her why you feel uncomfortable. Don't beat a dead horse either; I infer that you've been clear about your disapproval of her smoking in the past, so she may already know your stance on it. Focus on how it makes you feel, don't focus on defining what is right and wrong.
After she understands (and respects) your apprehension; try and find a middle ground. I can't give you a conclusive answer here, because I can't answer for your sister (or for you). Try coming up with alternatives, and try to achieve your sister's goal while also avoiding personal uncomfortability.
What will happen will depend on both you and your sister. If your sister isn't very sympathetic, it's likely that she'll be upset with you based on your initial refusal. There's not much you can do here, except explain your reasoning and express that you don't want to feel uncomfortable.
If your sister is particularly sympathetic, she might be okay with your refusal and doesn't need you to meet her halfway (e.g. once she understand your feelings, she may understand that she shouldn't involve you in her attempt to get cigarettes).
If it's somewhere in the middle, then you'll need to work out a compromise between the two of you. But at least you're now having an open and honest conversation, as opposed to misunderstanding each other and talking past each other.