I don't think it's right for the bride not to invite him. Yes, he's famous, and yes, it's an unusual situation. But at the end of the day, he's her sister's boyfriend, and the bride needs to respect that by including him. I do like the idea, if it's possible, of finding some way for him to meet the extended family prior to the wedding so it's not that big of a deal when he's there on the big day. It's natural that the family would be excited about meeting him, so letting them get it out of their system ahead of time could help.