Siblings are expected to be at each other's weddings.
Rob was invited to be a groomsman at his sister's wedding, but he doesn't want people to see how much weight he's gained.
Rob wants to be happy.
Rob decides to go to his sister's wedding and makes jokes about his weight.
Rob decides to skip his sister's wedding, because he doesn't want people to ask him about his weight.
Rob is happy that he gets to joke about his weight, and his sister is proud of him.
Rob is happier, but his sister is very upset that he skipped her wedding.