Wedding Issues.

So, me and my SO are getting married this summer. I'm having a difficult time picking my MOH and Bridesmaids. My situation is extremely awkward and just a mess. 
A little background.. Me and my SO have been together for 5 years. His Best Man is his cousin, who really is his best friend, so it's expected and I don't have a problem with it. I love him just like he is my family. His girlfriend and I have became pretty good friends over the past 5 years, we do just about everything together (as couples). But, SOs cousin cheated on his girlfriend a couple years ago, with... my best friend. Me and her were not friends at the time and I had no idea about it until a couple weeks ago when I had mentioned his name to her, while we were planning the wedding out. So now I'm stuck. What am I suppose to do? If my best friend is my MOH, she will be walking down the isle with SOs cousin and I just don't see that going over to great. But then it's my wedding and she's my bestfriend, and I want her there with me. I don't want to make SOs cousins girlfriend uncomfortable because we are good friends too but I can't pick her over my bestfriend. What do I do? Help! Please 😩 
I'm posting anonymous and leaving all names and dates out because my best friend uses this app as well, so I don't want her to know it's me in case she reads it.