Help!!

Just found out that an employee for my so is going to be staying with us for another two weeks. She's already been here for two weeks. I was told she'd only be here for two weeks and now it's changed to a month. Help so could loose job over her, but so doesn't seem to care. So is manager, while she is an employee. Plus, she keeps taking my vehicle without my permission, so thinks it's ok because permission was given by so. Have a gut feeling so maybe sleep with her, but can't prove it. What should I do? Tell the to f off and leave? Or stay and hope it changes? Have already tried to talk to so, but he could care less (or at least it seems like it)