So background - FH's mom only has one sibling, his uncle. Around the time we got engaged, FH's uncle had been with FH's aunt for 20 years, they have two kids together, FH's cousins. At the time we planned to invite them all to the wedding.
Fast forward to this fall, when FH's aunt decides to separate from uncle. We don't know the exact details or why, but suffice to say it was very unexpected for uncle. Aunt moved out and his cousins spread their time between them both (they're in their 20s). A month or so ago, aunt decided that she wanted to reconciliate, but uncle didn't think it was a good idea, so they are still separated.
So...now the awkward part. FH and his mom think it would not be appropriate to invite aunt. They want uncle to feel as comfortable as possible coming to his only nephew's wedding. FH's mom and he are very close. The only people aunt would know at the wedding are FH's mom and stepdad, FH's grandma, her own kids, and her ex husband. (continued in comments)