If you would ask me, I would say to be mevater. Godol Hashalom. Maybe there is some mistake, and will cause years of agmas nefesh for naught. You may not want to speak to them for a while, but the pain that family disagreements cause is usually to great to bear. You will think about it 24/7, and take up all your kochos, and get into a depression. No amount of money is worth it, IMHO.
There is a story about Reb ARyeh Levin in Tzaddik Yesod Olam in which a father was fooled by one son into mistakenly signing documents that cut his second son out of the will. The second son used to work with the father and take care of him, while the perpetrator had never helped the father at all, yet arranged to inherit all.
When the second son asked the father why he signed this, he admitted it was a big mistake, “I don’t know why I did it, and I will be glad to sign a document saying I was not in my right mind at the time, so as to nullify the will.”
However, the son said I can never degrade my father to have him state that he didn’t know what he was doing or was not of sound mind, so he decided not to challenge the will, and let it go.
Obviously, I don’t know the situation here, but keep in mind that it is a huge mental burden to go to Beis Din any time, and much more so if it involves a relative.