Speak now or forever hold my peace?

I could really use some advice. My FH and his siblings do not get along at all. At his first wedding his brother ditched him at his own bachelor party and celebrated without him. Things have not gotten better from there. When we asked him to be in this wedding he declined and then said no to the RSVP before deciding (28 days after the RSVP date) that he was going to come and made all the arrangements without asking us if we could even fit him in anymore. We did. However, this past weekend, his stepmother asked if we could squeeze in his other brother (that doesn't get along with FH or the other brother) and use our photographer for the wedding to get pictures of the three siblings. We agreed. However, since then FH has become very irritable and has completely lost interest in the wedding. I want to talk to his stepmother to see if we can cancel having his other brother come but I am not sure whether this would be rude. Any suggestions on how to handle this would be appreciated?