I don't have children, so normally I would not respond. But I would like to tell you about my SIL and her daughter.

Daughter got pregnant, had baby, moved back home, got pregnant again, boyfriend moved in, boyfriend couldn't hold a job...blah blah blah..it seemed like it wouldn't end.

Finally my SIL went out and rented an apartment, paid the deposit and first month's rent and gave her daughter the key, saying "I love you, but you cannot continue to live with me. Do what is necessary. Ask for advice if you want, but you cannot stay here. We are moving you into your new apartment on this date...."

This tough love worked.

I admire what you are doing for your niece, and I wish you the best of luck.