The first thing I would do is file a police report for the items that are missing. I'd tell the police the situation and ask them to investigate considering the fact that you have a serious suspect.
Then, I'd try and get a temporary restraining order against this nephew - the basis could even be assault, that you suspect that it was he who put the snake in your room and broke into your car.
Your query isn't clear. Who owns the house? Who's the tenant? if your brother-in-law is being foreclosed on and wants to evict you so he can get "cash for keys," if you're a paying tenant, you may l have rights of exclusive possession.
He can have his nephew as a guest if he wants, if he's the owner or tenant.
Try asking a question with more specific facts identifying the parties and their status.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.