WWYD Please help (a little long)

So I own a home with my husband and have an extra bed in one of the bedrooms.

My younger sister who is 23 and married got into a fight with her husband and moved out of the house and was bumming around with her friends while having my 5 year old nice stay at these places. She did not want to say where she was at and who she was with. She said her and her husband were done. And rumor was that she was seeing and staying with someone that she was crushing on.

Fearing for my niece I asked them to stay at my house so she could get back on her feet and figure out what she's gonna do. I would rather have my niece in a safe place than with random strangers. I had to ask my husband to please just give it a try and if she did anything wrong or made him uncomfortable then we would ask her to leave so he finally agreed to let her stay.

Fast forward a month and she is still here. Supposedly they stopped renting the house they were at and he's staying over at his mom's house. Now she leaves sometimes and doesn't say where and leaves my niece here with us without asking us if we can watch her. My husband and I have very stressful jobs and I just want to be home and relax not watch a 5 year old. But tonight was the cherry on top.

She is now hanging out with her husband again. Coming over to see my niece, I understand, but tonight he came into my home and went to the room she was in and the lights were off. I was so pissed off I asked him to leave. He was already taking a "nap" AT MY HOUSE.

What the hell is this?? So now I have about 20 days before my husband and I leave on vacation and I DO NOT want her here when we are not here. I believe that was so disrespectful and I do not know how to go about telling her she needs to find somewhere to go when we are out of town. And also when she's going to leave my house if she's still talking to her legal spouse again.

Feedback?