Katie, maybe set some rules and see how it goes.
Rules such as "You will be here when you say you will be here and not disappoint the girls"
Or maybe try some interaction between her and the girls with you supervising, to see how interested she is.
For now I think maybe playing it by heart is best. It would be nice if she stayed clean and could interact responsibly with her children, some can. But if that is not the case, all bets are off.
Just throwing out some thoughts. And some hugs because this must be hard for you.