First, let me start with an analogy. Have you ever had a sore knee? A really really sore knee, where you stop at the top of the stairs and take a really deep breath and have to steel yourself to make it down the stairs and you say ow with every step? I think most of us have, sometimes. Now, have you ever had a knee so sore you just couldn't face the stairs and asked someone else to go get you what you needed or got by without it? I have. I have recurring joint pain and there are days that stairs are just not an option for me. I have to ask someone else to help. Why am I telling you this? Because if someone I lived with were to tell me look, I'm going out, I put the laundry on, it will be done in an hour, I really need you to go down to the basement in an hour and get that laundry and take it up to the top floor and fold it, I would probably say ok, that makes sense, I will try to do that. I understand it needs to be done. But if my knee was bad when that dryer buzzer sounded, I wouldn't go down to the basement. I couldn't go down to the basement.
Bi Polar Disorder is not a sore knee. But it can keep people from doing things. From getting up off the couch and keeping their promises. From eating, or sleeping. And unlike my knee, when you feel terrible for not doing what you said you would do, you can make your BPD symptoms worse. I have a number of people in my life on anti-depressants and with problems with executive function. I also know, slightly less well, people on anti-depressants who have problems with impulse control.
What these people have in common is that explaining rights and wrongs to them will not help. It would be like lecturing me "when the buzzer sounds, the dryer has finished. And right then the clothes are warm. If you leave them there to get cold, they will get wrinkly. None of us wants to go out in wrinkly clothes. It's important to go to the basement as soon as the buzzer sounds." I know all that! It's not lack of knowledge that has kept me off the stairs! And the same often applies to people who are struggling with mental illness. They know they should do their share and help more, or clean up after themselves, or contribute towards bills, or keep their appointments, but when push comes to shove, they don't.
Now then, how can you talk to your brother about your issues with his behaviour? I recommend you stop explaining to him why he should not go out with friends, or should not spend money when he hasn't contributed to bills, or should help around the house more. He is an adult. He knows what he should and should not do, and why. I recommend you start being compassionate about his disease. For example, asking "do you think today is going to be a day where you could vacuum?" or "how much can you pitch in to the X bill this week?" lets him have more control than "could you X please" or "it's time for you to X." If he says he will do something but doesn't, you can explore with him what it was about the day and the task that kept him from being able to do it. Is the problem getting started? Perhaps we can start together. Is the problem a feeling of defiance that keeps him from doing something after you've told him to do it, even if he wants to do it? Is an optional part of the task (like driving himself to an appointment) keeping him from doing it, but if he got a ride he would do the rest? Does he have any understanding of what need he was filling, what itch he was scratching, by going out with friends or buying something? The phrase "retail therapy" is a natural opener to conversation about what sort of therapeutic needs he has. I have no idea what particular mental challenges your brother faces, and I don't think you do either.
If you are able to have compassion when he doesn't do things, or does things he shouldn't, and you are able to help him do things by meeting him where he is and helping him, things may get a lot better. That's what I've seen with my people. But I do remember an old acquaintance who was selfish, greedy, and lazy. He was diagnosed with depression and spent years in therapy and on medication. Eventually the psych said he was recovered, and what remained was just his own jerky, rude personality. Depression can make people act like horrible selfish jerks, but also some people are horrible and selfish underneath all that. I recommend you ignore that possibility for one simple reason: he is already behaving as horribly as you can tolerate. It seems very unlikely that compassion and helpfulness will make him do more unpleasant things, since he is doing so many already. But this approach may make him more productive, thoughtful, and helpful. It may enable him to control his impulses. It may make your life (and your parents') simpler and easier in addition to helping your brother.
One final thought: this is hard work. Depressed people are very frustrating. They don't argue, they say you're right, but then they don't do it. It looks like they are lazy and just enjoying you doing all the work. They rarely thank you for understanding or for helping. Other people tell you that you're letting them get away with stuff. It's a lot to take some days. But my experience is that people can get better, and when they are better, it's all worth it. No guarantees in your case of course. I'm just telling you my experience.