What is the federal law against opening mail that isn't addressed to you? And how can I approach the situation if I am a victim?

I currently live out of state, temporarily, and while my mother collects my mail and sets it aside for me, another family member takes them, opens them, and hides them from me. I know there is a law against opening mail that isn't addressed to you, and I'd just like to know how to handle this situation. I need some sort of formal arbitrator, because this family member and I do not talk at all. The mail this person is opening is official mail, transcripts, important information (not junk mail) and its beginning to be an issue since this person is hiding it, and storing it away from me. I'd like to take legal action, but I need to know what is the appropriate process to take, in order to resolve this situation. Please help me, I'm not sure what to do, or who to contact, or where I'm supposed to report this.