caught in between my eldest and my middle's girlfriend
I am the mother of 3 adult sons, all ranging from 28 to 23. My eldest is married to a lovely young woman, my second has been in a relationship for the past 3 years, and my youngest is single. My husband's family is very tight-knit, and we raised our children with a strong sense of family values. Our sons were raised very closely with their cousins, and we are so close that we have entire family-wide FaceTime calls once a week, on top of multiple international meetups, joint vacations etc.
I was very sad to hear that my oldest son (and I suppose by extension his wife) are in a feud with my middle child and his girlfriend. It appears to be that the girlfriend organized a party coupled with a family reunion weekend and didn't include my eldest and his wife. They invited my youngest son, as well as more extended family members, but not my eldest son nor daughter-in-law. From what I know the four of them have a very close relationship, and my DIL was very welcoming to the younger girlfriend, even offering to include her in her bridal party to 'welcome' her into the family. I asked my middle child and girlfriend and they claim they've never had conflict, which my eldest corroborates.
Anyhow, it turns out that this entire gathering and party happened completely behind the back of my eldest and his wife, and they were dismayed to learn that the girlfriend and the other relative invited were having sidebar conversations on how to broach the topic of this party/weekend reunion with my eldest and dil when they saw them. They said they found out about the party and the reunion because the girlfriend posted photos in the one of the family groupchats, and online. My youngest claimed he only seriously considered attending and traveling for it quite close to the date, and never asked whether his older brother or SIL would be there. Given that they are also very close - especially my youngest and DIL - I was surprised that it didn't come up. We spoke to our middle child and girlfriend when all of this came to light and told them they needed to make this right and apologize. We thought they had, but recently heard from our eldest and dil that not only had they not apologized, but that recent conversations between the girlfriend and my eldest had escalated into full-blown personal attacks. She defended her claim that as the organizer she only wanted to include the members of our family she felt closest to, and while she loved my eldest and dil as her own family, she simply has closer and stronger relationships with other family members. There is only a 2 year age difference between my two eldest sons, and it is true that the other relatives she invited were on the younger end of said spectrum, but again, all were 23-26.
My eldest is furious, saying he expect us as a family to stand behind him, and is asking that we retrace group chats to 'only include immediate' family since the girlfriend used it in the past to 'flaunt' her exclusion efforts. He also was clear with me that while he wants to fix things with his brothers, he is no longer interested in 4-way conversation channels with the girlfriend. I suspect my DIL has a similar position. What do I do? I feel caught in the middle and don't want to take sides. I want the 7 of us to be a cohesive unit and the girlfriend's family has been very generous towards our son, I suspect she will be in the picture forever.
tl;dr my eldest son seems to be in a feud with my middle child and his girlfriend. Ethically, I know what she did wasn't great, but I also know she is rather immature and likely wasn't malicious in her actions. I want all of my children and their partners to feel like a cohesive group and don't want my eldest and middle child to have individual relationships while cutting out a partner as it will mean we can't do group activities or vacations all 7 of us anymore.
edit for more info: my middle's gf's family has been extremely generous towards our son, they treat him to dinner all the time, take him on family vacations, etc. My niece (the one who was invited to said weekend reunion/party) is getting married this summer and everyone is invited. It had been our original plan to get an Airbnb for all 7 of us. I know my eldest and DIL will be cordial, I just want them all to be friends and close and not just superficially polite. I worry that anything we do that retraces a 'family only' line will be taken by the girlfriend's family poorly (ie creating new family only group chats, not including her in an Airbnb) I don't want to do that and do want to include her but at the same time it seems very clear she deliberately excluded my eldest son and is playing middle school exclusion games.