Is it possible to track an IP from an online form? To determine who made changes to an online account?

Also cross-posted to r/technology, if there is a better forum please let me know.

My uncle recently died (never married and no kids) and our family have been in a constant back and forth with his girlfriend regarding what she believes is hers or what my uncle “promised” to her. Much of my uncle’s very personal items, some of which are irreplaceable heirlooms or very valuable, are mysteriously missing or not where my uncle said he left them. With no real living will or other documentation of what he had or wanted to leave anyone until literally a week before he died, it’s difficult to determine what is missing but we are confident the girlfriend had a hand in it.

I’m fairly confident that we will never see those things again, but I am hopeful that at least some justice will be served for this issue. Shortly before getting very sick, my uncle had a life insurance policy through his employer that he wanted my mom (his POA) to check on. Turns out the beneficiary was the girlfriend, which was not correct. My uncle originally made my grandmother the beneficiary, and he swore up and down that he never made any changes to it. Fortunately it was fixed in time, but again it was fairly obvious who it could have been, if it wasn’t my uncle who filled out the online forms and endorsed a signature.

But now to the relevant question: Is there some way to determine an IP address of whoever made these changes and basically forged my uncle’s signature? Where do we begin with this? Thanks so much in advance.