Saying "no" doesn't automatically make you sound bad or selfish. If you are a people pleaser, or if you have been taught that it's not "nice" to refuse requests, it may well make you feel that you look like a bad person, but it's not a given.
If you haven't articulated your need for time alone and why, then do so right now. It should not be hard for any reasonable person to understand.
You might say,
I was more affected by my mother's death than I could ever have imagined. I am emotionally stretched very thin, and I need time alone to grieve. I would prefer not to babysit until I'm feeling better.
There's nothing in that statement that sounds "bad."
If you have articulated your need for some alone time, and this need isn't being respected, please read on.
Boundaries will make your life so much more habitable. Boundaries exist everywhere in the form of laws. You're not allowed to go into a stranger's house and leave with their TV. You're not allowed to leave the scene of an accident where someone has been injured. Laws are there for our protection, so that we can coexist safely and respectfully. There are consequences to breaking these legal boundaries. Personal boundaries function in much the same way.
People often say they set a boundary, but it didn’t help. There’s an art to setting boundaries. If it’s done in anger or by nagging, you won’t be heard. Boundaries are not meant to punish, but are for your well-being and protection. They’re more effective when you’re assertive, calm, firm, and courteous. If that doesn’t work, you may need to communicate consequences to encourage compliance. It’s essential, however, that you never threaten a consequence you’re not fully prepared to carry out.
Read about boundaries until you are sure you can not only set them but can explain why they are important. Your husband (and maybe your stepson) will push back. You need to remain calm and reiterate your boundary and why you're imposing one. Set a consequence and state it. Then walk the walk.
The conversation should take place with your husband and your stepson separately. when you're both in a good place mentally. It can go something like this (truncated for brevity's sake):
["Husband"] I love you, and I enjoy helping your son when I can. But right now, I am feeling emotionally drained, and can't babysit. I need you to know that I'm hurt when you don't respect my expressed need be by myself. I can't babysit right now, and won't. If you need to say yes to a request to babysit, do so, but please know that I will not come out of my room. If you get angry, I will leave the house.
Then walk the walk.
There will be push back, but if you're serious about being taken seriously, you need to assert yourself. You just have to believe you deserve certain privileges and be able to articulate them to people who are accustomed to treating you as if you don't.
Again, read more about boundaries. They will help you understand more about some causes of conflict in relationships and how to minimize what you can in healthy ways.
What Are Boundaries
Having a conversation to assert your boundaries