Sounds like your sister is taking advantage of you, as a free babysitter.
It is a tough job looking after an active child especially if you are having difficulties trying to care for yourself.
It is draining your energy and it sounds like you are dreading weekends, because every one is being on duty, so your sister can have time off.
Sounds like you may need to set some boundaries with your sister and tell her, you can’t look after your nephew every weekend.
You are a great sister
and aunt, just being taken advantage of.
Time to take care of you, so you can spend quality time with your nephew.
Sending you hugs! 🤗🤗

I'm very happy for you. Good job. I know it's difficult, but enlarging your self care toolkit helps immensely.

I feel guilty every day because I have two children 5 n 3 and there's days I am so fuckin annoyed irritable and exhausting to the point of not moving from one spot and I feel like I must be a big piece of garbage ass excuse of a wife and mom all the time bcuz they didn't ask to get stuck with the one sorry woman whos got major depressive disorder and generalized anxiety and serious lists of mental illness running strong through my DNA and my mom got worse with age the list is still growing and I know im only going to add on to mine to but here in the past month or two I've done forced myself to start pushing to give my kids and husband more out of what they were unfortunate to be stuck with and I've gotten more active not fully and every day I struggle and some days I lose but looking back I can say I've obtained a few more coping techniques that have improved on bettering how my activity and my interactive habits w everyone are and if I can say im not doing as bad and I'm more present than absent than I was last week or last month that's just enough to have me a fraction bit more motivated to do better tomorrow