Your sister sounds absolutely horrible and I'm sorry you had to deal with all her problems. It feels like the situation is repeating itself, because she's already trying to push her second child onto you. You've done way more than enough and I can't say it enough, you've already been a parental figure for her first child and your sister is being super unreasonable. She shouldn't bother having more kids if she's just going to dump them on other people. She needs to get her life sorted out and stop being horrible to those around her, beggars can't be choosers especially if she's treating the father of her kids like that - she needs the support and her kids deserve a father figure.
It's not your problem to deal with, you've already been a massive help which she won't ever be able to pay you back for and you deserve to focus on your life instead of all your sister's babies. I'm a uni student too and I understand how difficult this can all be and sleep is definitely important! If you carry on dealing with all her problems for her it's going to lead to you burning out and under performing at university.
You're not evil.
You're not wrong.
You're doing your best and your sister is ungrateful - she is not entitled to your help and shouldn't be forcing this all on you. I find it funny that she talks about helping you in the future when she barely deals with her own. If she's complaining to her friends about all this, then you can also let them know all the madness your sister has put you through, she's lazy, toxic and incompetent; you're better off cutting ties with her and let her experience all you've been through - she needs tough love as others have said. Good luck with your studies and I hope this situation gets better for you.