I don't understand the money aspect. Why is she not using her onylfans money to care for her kids? Is this some odd thing where only fans itself is paying for the vacations directly vs paying her money? Because if she's having income from OF that money should be spent on her kids not vacations. You are enabling your sisters neglect of her kids. So, as kindly as possible, I'll say Y T A for not standing up for yourself and your mom but NTA for now because there's still a chance for you to improve this situation. You and your mom need to put a total stop to the babysitting for weeks thing. Set a time limit I. E. 1 week, and call the police if she doesn't pick them up by then. Or ideally, don't even baby sit overnight in the first place, during the day only. Make sure your mom follows this too. If your sister can't stick to this then her kids should be taken from her, due to her neglect.

I know you probably don't want to insist on less /no baby sitting in case it impacts the kids badly, but in this situation you have to do the shitty thing short term in order for them to have a better long term. Yes, initially it'll be harder for them but long term it'll be better for them. Every time you babysit you're just side stepping round the problem, and delaying the process that could improve their lives