A couple of weeks ago we found out that his step brother won't be attending our wedding. We were told directly by his step brother and the reason he gave us was that his new girlfriend's friend was getting married the same day and apparently they sent out their invites earlier (and RSVPs back first), AND it was his birthday weekend and he wanted to spend it with his girlfriend (funny thing, it is also FI birthday weekend). He's known about our wedding date for a long time now. And he made it clear that because it was his birthday, we should understand why he chooses not to make it (he said that had it been another weekend he would have come to our's without his girlfriend).
FI is obviously hurt and upset because they are the same age and grew up together, were in the same class together and had the same friends. He considers him a brother and really wants him there. I don't know what to say to make it better or if I should encourage FI to talk to his step brother about it or not. I think his step brother should at least know how selfish and hurtful his actions are before its too late for him to fix it (if he wants to fix it) but I also don't want to push something that will cause familial problems.
Any suggestions would be helpful, even if you tell me to just leave it alone.