Ask Carl to lower her shorts.

"Carl, would you mind lowering my shorts a bit. Not much just a bit." Steffi asked nervously. She wasn't sure if this was OK for a brother-sister relationship, especially hers, but she felt she trusted Carl a little more today.

"Sure no problems" Carlo answered. He lowered it enough to get a good view of her ass crack, and just the top of her butt hole. At this point he became rock hard. Her butt was perfectly held up by her cotton shorts giving her a nice ass cleavage.

Carl started to rub Steffi's lower back more. She seem to be enjoying it but noticed Carl was grinding into her butt ever so slightly. She thought maybe it was in her imagination until she felt "that" poke her. But Steffi was so relaxed right now she didn't care enough to be annoyed.

What should Carl go for now?