This is your opportunity to show your sister's BF that YOU are not the problem. You know that she has been telling him what a bitch you are and this is your chance to show this person you don't know that YOU are not the one feeding the fire between you and your sister. Let him come. Make an effort to get to know him. Tell him when you meet him, "I'm so glad you're here. I know we tried to get together with you and sis for coffee a few times so we could meet you before the wedding and I'm sorry you weren't to make it but we're sure happy to meet you now. Hopefully we'll be spending a lot more time with the two of you." I understand your perspective on this and I would feel the same way in your shoes. BUT, this is the time you have to be the bigger person. I KNOW it's your wedding. I KNOW you only want people you are close to there. I KNOW!!! But you have to make the exception for this one. Your sister is being a bitch right now but she will always be your sister and you can't be the jerk right now. You just can't.