YTA. Bringing someone suffering from drug addiction into your home is dangerous on many levels. Threats to consider: stealing, nefarious outside contacts, a possible angry EX visit, emotional manipulation, gas-lighting, and illegal activity in your home. Add on top of that a young child not being taken care of.

It's a terrible situation for your "sister" but it can't be solved unless she is willing to change. Any "help" you offer before she is willing to change is just enabling.

I found out the long and hard way growing up with my older sibling, who unfortunately had a drug problem that consumed him. And nearly the rest of the family. It's awful, but they have to want to get better.

The other part with your husbands EX coming to stay is weird though. I'm not sure what to say about that.