Do you still have feelings for the older chick or something? What difference would it make? Things didn't work out with you two before so you're trying something new. Maybe you should of asked the older sister on a date when you were free? Idk. It shouldn't be awkward, if it is then it isn't something you should stick with. I can see it could be a little weird at times, but you and the older sister weren't anything official. She has a BF now, who cares what she is doing. Just see where it goes.