Off limits

Why don't you try getting caught on purpose doing something like jerking off to her or you could hide her bra when she's over

IMHO, if you are going to involve a female in your sexual gratification, make it fun for her. Women are not objects to stick your cock into, but people. If she gets off on watching her boyfriend's brother jerking off while topless, you're in. However, it's likely that she may take no interest since she's probably not getting anything out of it. I am uncertain why someone would even bring up "the conversation with your brother": I certainly wouldn't tell. And if his girlfriend thinks you even might, she'll say "no" without even considering it. People, but women in general, don't like their private, intimate lives revealed.

It's called imagination, there is no way on gods earth that your brothers gf would agree to this, and imagine the conversation with your brother