AITA for refusing to lend money to my sister even if it’s for my niece/nephew?

I'm a 25f and my sister is 38. She has five children, and one of them is already in college. I'm single and currently working. Sometimes, my sister borrows money from me to cover my niece or nephew’s school projects, which I don’t usually mind. In fact, I sometimes don’t even ask her to pay me back because I consider it part of helping them out.

But lately, it feels like she keeps borrowing every time I get my paycheck. It's becoming a regular thing, and I'm starting to feel taken for granted. Would I be the asshole if I started saying no, even if her reason is that it's for my niece or nephew?