Well, If you really feel like you wanna be with her older sister and can handle the possible catastrophes that COULD be involved. How much resentment is there in your breakup. The worse the breakup, the more problems you are going to have.
Basically, if it just didn't work out and you both kinda said.. "Eh... just not doing it for me." (Rare, but it can happen) Then, it might not be too bad. A little awkwardness at first, but eventual acceptence depending on your treatment of the sibling.
She dumped you, for nothing you did wrong, but she didn't feel it was working out. And you really DIDN'T cheat around on / screw over her. She can't have a problem with it. She doesn't hate you, but she broke things off. She lost her rights at that point.
You broke up with her for above reason. She'll be pissed you tossed her off for her older sister, but since you weren't a dick to her (or about your breakup) eventual acceptance would most likely result.
Angry Breakup - She busted you doin sumtin bad. Major headaches. I think you can guess where this leads to. (You cheatin bastard)
Angry Breakup - You busted HER doing sumtin bad. Major headaches, but you're in the right and fuck her and her lesbian lover for not letting you join in. (Or whatever......) Warning: Her sister will have been most likely brainwashed unless she was personally aware of the situation. Might require intense groundwork.
or
You broke up with her because you found out her lesbian lover WAS her sister, and now you're trying to hook that shit up, cause you realize what a dumbass you were for backin out of that one.
Good luck!
*Edit*
Oh yeah. I broke up with a girl a few months ago. We were just meant to be friends (and we are now) But her best friend had had a boyfriend for over 2 years, she broke up with him. I really like her friend, but I never thought about it at all when I was with my girlfriend. (Not seriously anyway :O ) because she was in a relationship cause I don't mess with another guys girl. Hell, if she'll do it to him, why not me. And I just don't want the problems that would be very HEAVILY involved in that kinda situation.
So I had broken up with my girlfriend long before those two did. And after about... 7 months, I find out that she had broken up with paul and was dating a new guy. Hell, the guy had the same first name as me.. (That just stings) And, I just RECENTLY found out that they've broken up.
I really wanna ask her out, but.. like.. I'm not sure how my ex would take it, and she really is a good friend of mine. I wouldn't do anything to hurt her feelings.... I'm gonna see what I can do but... arrgh