I have to agree with DZrose, obviously theres something else to the story that your sister isn't telling you, maybe she did want a relationship with him and it was just not possible or maybe she's just embrassed about seeing him there. But she is an adult , she made the decision and they both have moved on. So since you can't take your boyfriend, why shouldn't you be entitled to having fun ! I say take him and have fun. HOpe everything goes well
DGPhoney~