Is it possible to legally evict a family member?

It's certainly possible to legally evict a family member, but if he's moved out and isn't living there an eviction proceeding isn't necessarily going to get you the results you want. An action for trespass seems like the more appropriate mechanism, whether enforced civilly by you or criminally by law enforcement. Of course, there are costs and certain effects of proceeding with a trespass claim, so you'll want to be sure that you review the situation with an attorney and go over all the case details with them. Often, these situations are best left to the individual family members to work out amongst themselves, but if you've tried to find a resolution and have been unsuccessful, an attorney will be able to help.