Yep ... thought I had seen it all .. this one moves to no. 1 revolting for me and dare I say of no interest to anyone here on Lush (just my personal opinion of course).
Having a pantie fetish myself, I don't think I would. Unless he didn't leave some for me too!
You should get drunk and have a word with your friend - you'll probably find that he was wrapping his sister's underwear around his cock when he masturbated and then throwing it in the laundry.
Omg I feel the need to throw up. That is the weirdest and grossest thing I have ever heard of someone doing. I really hope you never told her your little secret.