Both of my parents had issues with inheritances when their parents died. Assets disappeared, siblings banded into factions, and in some cases they took grudges to their graves. Sadly, even with these immediate examples in our recent family history, my two sisters are waging a war over their future inheritance (or lack thereof). I know there are two wills with different provisions (mom's and dad's) and I know there is a new will (dad's) which was drafted in secret with consultation from one sister only, who is pushing hard for it to replace a previous will (dad's) so that no one but she (and dad) will know the contents. Also as mom's health is failing, the same sister is actively moving assets from mom's name to dad's, presumably so mom's will is mostly superseded by whatever is in the new secret will she is pushing through.
It sounds like a soap opera, but I can tell you with certainty from family history that people do irrational things, even at the risk of alienating close relatives when money is involved, and inheritances seem to bring out the worst of these greedy behaviors. Even when the sums are small, people sometimes do amazingly viscous things to family over money. In this case, the assets are substantial enough that I don't expect the sisters to mend fences. On top of that, it is likely that moving assets is going to break some of the tax planning that went into the original wills. My brother and I are hunkered down and trying to be friendly with both sisters. I don't need the money and while he could really use it, I think he genuinely doesn't care about it, unlike the sisters who seem to think it's some kind of validation of their place in the family whether they need it or not. I hope we will be able to maintain family relationships however this plays out, but I fear the two sisters may be setting themselves up for unhappiness, and maybe an open feud, no matter what happens.
Beyond this, I cannot think of anything I can do to help this developing problem. I certainly don't want to take sides. I tried gently suggesting mom and dad should do whatever they want and even that as I am in reasonable financial shape, leaving me out of any distributions was fine by me. Unfortunately the sisters treat any suggestions as support or opposition to one or the other of them, so no suggestion can meet with mutual approval.
