We always spend christmas with my parents. Me and DH, our 2 DSs, Dsis and her family and DB and his. There are big age gaps between us siblings and therefore the cousins.
Since kids came on the scene they've been at a separate "kids" table for reasons of space and my parents being OTT about "mess" (plastic tablecloths, no alcohol, kids crackers etc)
My sister's daughter is much older than the other kids and has selflessly put up with this for years and we've always had a little private joke about how silly it is. She's nearly 22. This year, my DB (aged 40) split with his long term partner and is planning on bringing his new girlfriend, aged 25...
My DN was a bit 
about all this, being quite close with DB, but has lost the plot about the ongoing table arrangements. I think she's probably projecting anger about other stuff onto this situation (loss of DBs partner who we all liked etc) but she is threatening not to come if new GF gets "treated like a grown up" while she's subjected to a boozeless christmas dinner with the little ones.
I am actually quite supportive of her- there must be a more inclusive way to do it all- and have said so to DPs who are having none of it. This silly thing is threatening a bit of a rift- me, DH, DSis and BIL and DN all on same side with the rest of them on the other. Suspect DBs new GF has no clue.
AIBU to support my DN in this admittedly a bit silly fuss? How would you resolve it?