Sorry for just jumping in now, but I just saw the title of this post in my "unread posts" page.
I have a brother who used to steal my panties, along with my mom's and older sisters. I hated it, it was gross, embarassing, humiliating, and made me very very uncomfortable to be near him. He'd come in, sneak into our drawers, and steal our stuff. I thought I was going crazy thinking I was somehow misplacing my underwear until one day I no longer had any panties or bras left, so I told my sisters and she said all of hers was gone too. The same with mom. Turns out, brother was stealing them, and sticking them in his boxers and masturbating with them.
Sorry, I think it's completely wrong to go and steal someone intimates, whether or not you're using them to get off with. Panties are cheap if you get them in packs at Wal-mart. I don't know how old you are, but save up some money and buy your own. if you keep stealing your sisters clothes, you could potentially be hurting any future relationship of trust and friendship with her.
Edit: I also just can't understand the commenters who say "it's not wrong, just don't get caught". Do we say it's okay to rob a bank, or kill someone, just don't get caught? I thought family relationships were about love and trust and you can't get that by going behind someone's back and stealing or "borrowing" their intimate apparrell.