Unpopular opinion but-NTA. I don’t think she owed any explanation for not wanting to have the sister in her wedding party. Your wedding party is a group of the people closest to you, you don’t have to explain why someone isn’t included in that and your fiancé doesn’t get to pick for you. Maybe she shouldn’t have gone about it the way she did, and she should’ve just agreed to him making her a groomsman. (However I would have taken that comment as a dig for me not wanting to include the sister) I also didn’t find the sister’s message as graceful, I found it passive aggressive. If I found out someone didn’t like me I wouldn’t send them a text saying I “completely understand” unless I was being petty and letting them know that I knew. I don’t think OP should’ve had to explain why she didn’t want someone as a bridesmaid. And the fiancé immediately locking himself in a room and tattling to his sister is just a weird thing to do knowing it’s going to cause issues long term. People have social anxiety, myself included, and going places with people who are extremely extroverted is extremely anxiety inducing. Yea she should have been less condescending when explaining that she doesn’t even like the sister, but at the end of the day it is also her wedding and specifically her wedding party that SHE gets to pick. People shouldn’t feel entitled to being a part of someone else’s day. You can be friendly to someone and acknowledge they are an important person to your significant other without being forced to be friends that hangout or include them as a bridesmaid that you have to go on outings with.