Estranged Father Invited to the Wedding

Question: My best friend is getting married and her parents are divorced. Her mom has never really gotten over the relationship (it's been 15 years) and, along with her two siblings, harbor resentment towards him. My friend has recently started speaking to him again (after years of him being out of her life) and is the only one working on possibly rebuilding their relationship. She wants to invite him to the wedding, but her sister is saying out of respect for her mom, she shouldn't. I told her that I think she should invite him and that her family should put their differences aside for ONE day if it's what she wishes. Her sister is saying that per etiquette "rules" she shouldn't invite him if it makes them uncomfortable. I disagree - please help!
Thank you!

Dear Megan,
You are correct, the bride's sister is full of crap--she's obviously taken her etiquette lessons from the "Politically Correct Whiners" school of etiquette. It is the bride's day and if she wants her father there, then everyone needs to put aside their animosity for the sake of the bride and be polite to each other out of their love for the bride. Her sister is incorrect about the etiquette rules and needs to be given a copy of an etiquette book so that she can learn the rules instead of making them up to suit her wishes. The rule is crystal clear in this situation and the bride is the one who gets to have it her way on her special day. My sister used to have a 'second seating' and two Thanksgiving dinners so that my parents didn't have to see each other. When I moved back from Japan and had Christmas at my house, my sister was totally freaked out that I wasn't having segregated parties but I refused to do it. I told all parties that it was MY home and if they couldn't behave like mature adults that the offending party would be asked to leave. They behaved beautifully and now the entire family gathers and everyone is polite to each other. Your friend needs to speak to each of her parents in a firm but kind way and tell them that it is HER wedding and that she EXPECTS them to make the effort to get along with each other and if they do not, then they can leave the ceremony and/or the reception. They will behave--but she needs to be firm and not cave in to threats or emotional blackmail--i.e. "if your father shows up I won't be there". If her mother says this, all the bride should do is say "I'm sorry you feel that way, we'll miss not having you there to share my wedding day." and then she needs to turn and WALK AWAY. I promise Mom will show up--and behave.