Could you perhaps...get in touch with the people at her rehab and discuss options with them? Perhaps they can direct your sister to a good sober-living facility. Once the resource is in place for her, you can announce that you will not be allowing her to live with you "for the good of everyone involved". She wouldn't be on the street AND she'd be in an environment that would be supportive of her recovery all the while furthering her chances of obtaining visitation with her children.
Of course, she could tell you to f*** off and do whatever she wants, but perhaps having her kids hang in the balance would sway her enough.
Methinks that this is a perfect time for you to establish and enforce your boundaries.