Possible Estate Shenanigans

My father in law passed away and had a few investment accounts held at a major financial firm. He had designated beneficiaries on the different accounts and my wife, who he was closest to, was named the sole beneficiary on the largest one. He was closest to her and we had struggled financially from defending a years-long malicious lawsuit on our small business… so I think he wanted to help her most…. but her sister (the eldest sibling and the executor of the estate) “found” a document from this financial firm that he had filled out but never submitted, which would have changed things to split assets evenly three ways.

The sister/executor was somehow able to get the beneficiaries changed based on the document. I did talk to him once about his estate as he was unsure how to leave things, and I didn’t really give him any advice just listened. So even if this document was legit it very likely may have been something he considered but decided against. He was up there in age but lucid, sharp and very detailed up until his passing so I have to believe he had things the way he wanted them.

It just seems highly unethical/shady to us that the financial firm would make this change posthumously. I would greatly appreciate any advice/insight about this.