Help with nosey sister-in-law!?

Is there a way to send letters or a package to someone and to make sure only they can sign for it? My brother and I are really close he lives in a different state. When I send him something my SIL always opens it. I've sent him gift cards and then found out she uses them. How can I get something to him without her getting then first. He works at home so I can't send them to his office or anything. Please help