Wow. That's just so many kinds of messed up on your brother's part if he genuinely tricked your BF into having sex with him. As in, he could be charged with rape levels of messed up; and even if he can't be charged for whatever reason, it still was rape. You might want to discuss things with your BF about setting up a way to keep your twin from doing this to him again. This almost sounds like your BF had to have known something was off, except it'd be a special kind of dumb for the alternative to have been the case.