When. I was first teaching, I was a pushover for parents wanting to dump their no longer wanted pet hamsters on me.
I had a great deal of difficulty saying no.
As a result, at one point I had 3 classroom hamsters.
Three separate cages, trips in during breaks to feed and water or bring them home with me.
I was moaning about this to an older teacher who had been in the classroom a long time.
She smiled. Looked right at me, and said, "sometimes we have to learn a lesson more than once."
Meaning, "Sharpen up your NO skills, kid."
Sounds like you really, really want what's best for niece, and she just can't get it together.
Fwiw, this is my opinion only and totally not judging, as I have a sib who has been living with and off my mother for years, but....
When your niece moves out, I would try real hard not to let her back in.