I'll just add here that it's (obviously) none of my business what these people do or don't do.
I just really want them to be safe.
And in my humble opinion, shelters aren't necessarily particularly safe.
I suggest as an alternate option that the poster's younger sister (and her child with her) consider going to live with (or at least temporarily go to stay with) other (and preferably safer) relatives.
Hope everything works out okay.
Hugs. 🤗