Future stepchild does not want to attend his father's remarriage?

My sister just became engaged. Both she and her fiancé have kids. The one child of her fiancé does not want to attend the wedding. He is still very angry over his parents divorce two years ago. He has never wanted to do anything with my sister and her kids, and he wants no part of attending the wedding. He’s in his teens, just like the other three kids. My brother in law to be is a good person, and has been a good father to his kids and kind to my sister’s kids. He feels upset that his son might not attend, and is pushing to have none of the children attend if all of the children are not attending. My sister is upset because the son is the only one unhappy about the wedding. The three other kids are either happy or neutral. She worries that if the unhappy son is forced to come the event will be ruined. So, no one knows what to do, and we thought some of you may have experienced the same situation and have some ideas.