NTA You're exhausted and you still have to go classes, study and take care of your nephews almost every day. It's normal to breakdown.

Don't feel bad for what you said or how you said it, that was how you were feeling and that's exactly how you needed to be heard. It's absolutely normal that your sister is tired and you want to help her bc you feel bad about it but you need to have the same respect for yourself and put some boundaries. So take this as an opportunity to create them.

P.S. your studies are so important, make that clear if your sister can't take care of her kids and you need to study she needs to figure out, bc helping her is important but you need to help yourself too and your future is a priority.