Yes. It is against the law to tamper with the mail. Your best bet is to report it to the office of your local (at your mom's house) postal inspector. This will take care of the criminal side.
If you have suffered financial losses because this mail is being hidden from you, then you need to speak with a local attorney that can help you recover your costs.
The simpler solution might be to file a temporary change of address with the Post office. They will automatically forward your mail until you go back home. Just a thought...
I would doubt that taking legal action or referring the matter to the postal inspector for criminal prosecution is the wisest course of action to resolve this apparently intrafamily problem.
Why not have your mother, in addition to collecting your mail, also hold it for you in a place that will be safe from this other family member allegedly involved in the surreptitious predation of your mail?
Michael E. Hendrickson
Attorney & Counsellor at Law
211 North Union street Suite 100
Alexandria, Virginia 22314