When my husband was in rehab and I spoke with an attorney about how to keep him out of the house, I was told the only way to do that was to start the divorce proceedings. I should have listened to that advice.
It's nice of you, to help your sister out - but really, her husband is probably thoroughly fed up with her and has every right to be. And, if she's drinking she has no business being in charge of children.
It might be that you'd be better off (if you feel compelled to help in some way) finding out if there is an Oxford House in the area for her to stay at after rehab.