I think it is strange that a 10 year old boy was invited to a 6 year old princess birthday party at all. Personally I would only invite guests that were similar in age and interests for a party like that. If I knew or wanted to invite other guests (older boys like your son for example) I would plan activities for all ages/genders of guests to enjoy, or have alternate games for the "big kids".
I think it would be fine for you to RSVP no for your son next year. It doesn't sound like he is super close to the birthday girls anyway. Is every family invited to these parties, not just the friends of the girls?
I'd have RSVP'd no for him this year!
Even if Dad is godfather, that's no reason for Son to attend his "cousin's" party with her friends. In fact, I'd say that I think it's weird to invite Godfather to the party with a little-girl theme, and I wouldn't go. I wouldn't even take my 7yo son to a party with such a "girl" theme. I'd find a way to celebrate my godchild some other time.
I think people who are in the position these girls' parents are feel that "have to" invite a wide range of people to "the party." (they have only one)
I refuse to invite family grownups (especially friend grownups) to my kids' parties with their friends. My MIL hinted and then flat-out asked, and I said, "Sorry, no, this party is just for their friends. The only reason I'm going is because someone has to run the games and get the refreshments out."
My kids had, what, four?, celebrations for every b'day
1) at the family dinner table on the actual day
2) at school on the school day closest to the actual day
3) with friends on a weekend day as close as we can get to the day
4) with ILs (and any other family members) at their house on a weekend night close to the day.