Dear Prudence,
My sister has never had a romantic relationship that ended well. Most recently I objected to her dating my fiancée’s brother, but she told me it was none of my business. A month before my wedding, they broke up spectacularly. I really don’t care who cheated or who got drunk with whom—I am tired of it spilling over into my life. My brother-in-law has been very quiet and personally apologized to me. My sister got drunk at my bridal shower and picked a fight with my sisters-in-law. My mother and I laid down the law; my sister sulked. She burst into tears and said that we didn’t care about her broken heart. I am ready to tear my hair out. My mom tells me to give my sister “time.” But she only dated him for three months. I have been planning my wedding for three years! We are paying for everything ourselves. My sister and brother-in-law are both in the wedding party. Can I just ban my sister for my peace of mind?
—Bridesmaid Brouhaha
Re: Break-up has turned my sister in to a bridesmonster.
Obviously the LW shouldn't kick her out of the wedding. What a terrible idea and thing to do to one's sibling. And, while I would also inwardly side-eye the major angst over a 3-month relationship, it's not for the sister to judge. If the sister says she is heart-broken, then that's how she feels and the LW should be sensitive to that. Just not at the cost of the sister mistreating other people and/or causing scenes.
But you can simply move on and tell HER that she's in charge of herself and making a spectacle means she's removed herself.
Don't have a head table, don't force group events and move on. When I was in BIL and SIL's wedding I wasn't hanging out with the GM. It wasn't a thing. So don't overthink the wedding party portion but DO tell your sister that your BIL is going to be your family too and she's got to learn to be civil in future family situations.
The only question is the sister going to stir the pot and put Sister with FBIL for the processional and recessional...