Wow. A lot of people are taking umbrage with the 15 year gap. I mean, it's definitely not a small gap and there are a lot of real obstacles to it working out, but I don't see it as some taboo dealbreaker.
Little sis is in love to share love, and intends to obtain her happiness herself. That's a mature enough line of thought.
While I can sorta understand the family's stance-they'd rather she not take a path of life that's full of hardship-the way they go about it isn't really helpful.
Mom is okay, but dad is...dad has basically become an obstacle in of himself, and he has crossed the threshold from trying to keep her from getting hurt into stifling her life to the point of potentially hurting her.
Also, I haven't forgotten what utter shits they all were arguing with each other right in front of a child as if he was just an unwanted burden. He's certainly not a child of their family, but when you're that callous, I have no reason to take your familial concern seriously anymore.