sounds like you've almost psyched yourself out of it.
Think of it this way. Because you're not that close the fear "eewww it's my brother" is further from her mind than most girls. This means she's going to come in with a more normal mind.
When you tell her about the no bra thing don't do it creepy. just say "by the way, I'm not some prudish ass that's gonna flip out if you slob out a little. As long as you don't mind my boxers once in a while I'm cool with you chilling in bra and panties. If you're really nice to me I'll give you time to change before I let our friends in. lol "
she'll see you have a sense of humor about it, that last line takes the focus off it, and you must be wiling to wear boxers (which are like shorts anyway).
Another way to be non-creep but direct is to say when she's in bra and panties sometime "god, why would anyone break up with you, have you seen yourself in the mirror? (walk her to a mirror" and say "you've always been cute but look, you have an amazing body? what the fuck was wrong with him that he'd walk away from this? lol
make a joke out of it, start referring to him as her blind ex-boyfriend, this becomes an inside joke but everytime it's used it reenforces the fact that you know how hot she is.
Dominate her time, plan times to hang out, go out, and then tell her things like "hey, walk closer, these guys said my girlfriend was hot, if you don't stay close they'll think I lied when I told them you're my fiance"
make a joke of that for a while and do things together, cooking, always keep the time you're spending together unscheduled and spontaneous so she gives up trying to make plans since you'll have things you want to do with her.
then eventually she may throw herself at you. probably sometime before all this.
