Weddingbee boards

So my fiancee and I have already decided who we want in our bridal and groomsmen parties. His best friend is going to be a groomsmen, but his parents absolutely hate him.

The problem is that they won't let it go. They say that his best friend can come to the wedding but can't be a groomsman. 

We've tried to explain that he won't cause any stress and won't even say a word to them. 

The reason they don't like him is because 4 years ago their son, my fiancee, and his best friend had to kick out my fiancee's brother from a volunteer program for acting inappropriately. So now they still hold a grudge against it and how my fiancee and his best friend should NEVER have done that to a family member.

Anyways, they say that my fiancee and I are picking his best friend over family and that they are going to leave the wedding early if he's in the wedding. They will not stop talking about it and are making us feel so stressed. We've told them we are stressed and that we don't want to have to choose, but they won't budge. His brother, who is in the wedding too, doesn't even care about what happened and says it's fine. It's just his parents.

All I want is to have my fiancee's parents to be happy on that day and for his best friend to be there. We've already told him he's in the wedding so we can't even try to not have him in it now. 

I guess I just don't know what to say to make the situation any better. Any ideas of what we should do or say? 

I'd appreciate it. Thanks everyone..