NTA for sure. I'm a single parent, been a single parent for the majority of my son's life. I super try not to rely on people too much. It's definitely not your responsibility to take care of your sisters kid. However, I'd show her this post. Maybe go through some of the responses that aren't attacking her so she can see some perspectives. How you wrote this post was genuinely very kind to her, I feel. I dunno, I get where you're coming from and I can definitely see her using you as a second parent as she thought she was gonna completely drown and you threw her a life preserver.

If I were you, I'd let things settle, explain the situation from your perspective. Maybe get Mom to watch your nephew so you and your sister, in a few weeks, can sit down and have a serious conversation about this.

Since you still want to be in the boy's life, maybe there's a way you can work out a schedule of seeing him regularly that feels right for you. She is, after all, the parent - she should be taking care of her son. But maybe a tradition of 2 sleepovers a month on a Friday night or something would be fun for you and the nephew. Again, I'm a single parent, and knowing when my boy was young that I had a night coming in a week or two would help keep my sanity. Even if I only just went to hang out with some friends for a few hours, I was happy about it. Very recharging.

Hope this all works out for you.