I am the oldest of 4 sisters. I live in DC, while all my sisters live in our hometown, more than a day's drive from here. Our father lives in Florida, and has limited income and mobility. I'll call my sisters Sherry, Terry, and Carrie.
Carrie, the youngest, is on Disability, and so is her live-in boyfriend. He has kids, who stay with them part-time. Both of them have had hard times AND made foolish choices. They're barely scraping by.
Terry and her husband have a kid. He has another kid who lives out of state. I don't even think he has any formal visitation, and the other kid almost never visits. I think our dad has only met this child once. Terry works very hard and is ALWAYS on her high horse about it. She bitterly accuses Carrie of being too lazy to work.
Sherry and her husband have no kids together, but he has a kid who visits weekly.
She isn't talking to Carrie because a few years ago she rented a house to Carrie and her boyfriend, and it went sour. I'm not totally clear on what happened, because I wasn't there, and they both start frothing and ranting when you ask about it.

To me, it seems like there were misunderstandings and stubbornness on both sides.
FWIW, Carrie and I are generally more easygoing, while Terry and Sherry are more hot tempered. They are "besties" now, but ironically,
they didn't speak to each other for a few years. Sherry went out of her way to help Terry with a big problem. Sherry expected Terry to fall over herself in gratitude for the help, but instead, Terry acted like an ungrateful snot.
Our dad is not healthy enough or wealthy enough to come visit. Every Christmas, he sends the grandkids gift cards. Yesterday, he sent us a group text saying that they were in the mail. I was in a meeting at the time, so I missed the "blowup." Carrie responded to Dad with a thank you, then for no reason, Terry attacked Carrie, asking why she was responding, when she (Carrie) doesn't have any kids? She continued, saying that she had a "real" stepkid who doesn't get a gift card, so why do Carrie's boyfriend's kids get them? She sniped about them not being married, and not having jobs.
Carrie sent swear word laden texts in reply. She got on Facebook and announced she was giving up on "certain people" and hoped they died!
Dad said it was his money, and his business who he sent it to, and asked them to stop arguing.
Sherry told them to show a little class and show Dad respect. She then started (privately) texting Carrie, reaming her out over the texts, the FB post, AND the old stuff about the rental house!
Now Carrie is "dead to" Sherry and Terry, and vice versa.
For Dad's sake, especially, I feel like I should try to mediate. I'm not in town, so that's one issue. More importantly, I (and others) have tried to make them see sense before, and it didn't work!
I hate to think the next time they're in the same room might be at our father's funeral! Is there anything I can do, here?