My sister in law (recently) knows this woman who has had many issues with her mental health and stability in life. This woman also has 3 young children, is single, and has burned every bridge she’s ever had with her own family. She just got out of treatment for the third time. Her son found her twice after she overdosed on pills. She is not kind at all— I know because I’ve spoken with her on the phone twice. She’s constantly nagging her kids about everything, and trying to be a parent when she literally does not have the ability to be. She has left her children with three separate homes in two different states— as one family is unable to take all three kids at once. So now, they’re all separated and miserable. 
I’m sad to finally say this, but this woman is a leech. She has explicitly said that she’s glad my SIL has the money to take on one of her kids, and constantly “jokes” about how her son will soon be one of their kids.
She says she needs a place to stay for one night this weekend. I told my SIL that it was a bad idea. This woman is rude, and has no problem using other people. I’m worried that she’ll guilt my SIL into more frequent future stays, and this could become a major problem. My SIL is EXTREMELY giving and kind hearted, and tried to turn down this woman’s request for a place to stay THREE TIMES. This woman finally broke my SIL down and got her to agree to one night’s stay.
My SIL is so stressed about it already. I tried to tell her to tel the woman that she could no longer stay the night, but my SIL says she can’t do it.
I need advice on what to tell her. I’ve tried to tell my SIL to set boundaries. She does, but then eventually let’s this woman break her down. I know she’s a grown woman, but I’m so worried she’ll be blindsided by this tornado of a woman who will drag her down and take further advantage of her and her money. Help me figure this out.