Dude, she obviously wants it, is putting herself out there waiting for you to get some. So
"Hey sis, you in the mood for some victimless crime?" you ask. She'll know what you're talking about and you can gauge her reaction. If it looks bad, or even if you just chicken out like a bitch, you can always play it off like you were talking about smoking some weed--who's the victim there?
More likely she'll say "OOoo what did you have in mind?" with a sly suggestive smile, which you can turn back to her
"That depends, what are you up for?"
Or instead of beating around the bush you could just start by smacking her ass, commenting on her hot body, ask if she wants to fuck, etc. BTW, who started the whole TABOO discussion in the first place, hmmm?
