Here is the thing. If she is going to continue her relationship with these unstable people, she will be as unstable as they are, as shown by her choices. It's her choice to spend, or not spend, her money. It's your choice not to be manipulated, and that's a great choice!
Of course you should not let this toxic mess of a person into your home. That is your safe place, and letting him in would be unsafe and cause problems at your own home, which you don't need.
You are not responsible to your niece, or to this man. She has resources, so does he. They have caseworkers they can reach out to. If they do so, or not, is their choice.
Huge hugs.