Dear John (OP),
Froom my heart, I must share wiith you that my own mother married a protestant, in a protestant church…my grandparents went ahead and attended, grandpa gave her away, etc.
Well, in a few years comes a little baby (that would be me) and the grandparents saw fit to pack me along to mass here and there, as they could.
As an adult, I went through RCIA and confirmed. I don’t think I would have found the Church if it weren’t for the relationship with my grandparents. Though they passed away and my last childhood attendance at mass was age 8 or 9, those powerful memories stayed with me.
Folks are free to lambaste my opinion that he go to the wedding. But I would ask they bear in mind the spiritual needs of those tiny little granddaughters. Grandma and Grandpa can help where mom and dad won’t.