Dude heres what you do,
because you've been hangin out for ages and know each other really well, this means that she already has a pre-conceived notion of you and see's you in a particular way!
You can't change that by trying to "convince" her that you'll treat her good and that you'll make a great couple...blah blah blah, all that nice guy crap because she sees you in a certain way and with the female psyche if your not a sexual being, then she shuts off that particular level of communication from you, and it stays that way. Man you should know buy now chiks dnt dig the "nice"guys,
What you have to do is take on the characteristics of "S.A.M" (the Sexually Aggressive Male) as pointed out by David D. And by this I mean to be more of of a cheecky and cocky sexual person. But since she already see's you a certain way, as you said yourself, she's like your sister so she doesnt see you as a potential sexual partner.
heres how you solve the problem:
Style gave this advice to a guy on one of his radio phone-ins,
This guy had lady friends that were really attractive and had all known each other for ages, but they only saw him as a friend or brother type(just as in your situation, except he was after multiple chicks),
so Style tells him to go out with them and let them see him interacting and being sexual, flirty and cheeky with other women, and once they see this happening they will eventually change their perception of him and will want to "compete" for him. Style tells him to meet new women and bring them over to introduce them to his lady friends, as this will escalates those competing drives.
Burton, you know the situation better than anyone else so you know whats best, but i strongly reccomend you do as style says and let her see you interacting with other chicks, this will change her "brotherly" veiw of yourself then become more of a "S.A.M" character. and as for her boyfriend...well if she you do your stuff right and she really digs you then she wont even mention him.....as far as you know he doesnt exist!!
good luck, n let us know how u go!
Regards
magnum-1