Since there is a question, I will answer.
It's not your fight, not your business.
People argue, they have different points, and see the other side's grievances differently than they see their own. Third parties are just choosing either on subjective merit, virtue signaling their bullsoykaf and making a mess out of it. If it doesn't threaten ending with something irreversible, say, a divorce, don't feel like you are expected to take sides in not-your-feud.
Whatever it is all about, you will see things differently in a couple of years.
I would invest myself in family issues that did not start out as my business, throw in my own emotional business and grievances into it, and the way it ended up, it was all anti-father meme. The situation was fucked up and my involvement fucked it up further. In retrospect, I see nothing beneficial or just about it, and I wish I would have just tried to stay objective, instead; even completely withdrawn would have been better. I needed time and a different perspective to even have the capacity to understand it fully.
The only good thing about it is that no one died along the way, so it all worked out in the end. Things got scarred, though.