Boyfriends mom invading my privacy???

Okay so... We just recently moved into my boyfriends moms house to save money for the baby coming. I have finally gotten my addressed changed for all my mail, so now it comes here. She laid my mail out for me on the counter which was great, only problem was... ONE WAS OPENED??!! It was the paper laying there without the envelope. I found the envelope in the trash. Why on earth would she open my mail and what do I do about it? I want to keep the peace because us being able to stay here is nice. HELP PLEASE!!!:(