I'm not justifying what your brother did. But I would like to give you the male perspective: when a guy with a normal libido is between the ages of 13-20 (something like that), they're incredibly horny all the time. Most guys release their sexual energy with masturbation or with a girlfriend (ideally). It seems patently obvious that it wasn't enough for your brother to *just* masturbate by himself. It looks like he was really chasing after pheromones, which your underwear had plenty of. There are sub-reddits where women literally sell their used undergarments, and they make a killing.

THAT SAID, what is fucked up about your brother is that he wasn't turned off by the fact that he was masturbating into his sisters underwear. That's like a huge red line that most men, no matter how horny, would not cross. Your brother sounds like he could use therapy because his perversion is really gross.