My Estranged Dad Kept My Sons Inheritance From My Grandmother A Secret.

So I just got a letter in the mail from a lawyers office stating that my cousin is suing my dad because apparently my grandma left her great-grandchildren money when she passed away back in 2021, including my son, and my Dad has ghosted everyone since then. I was very surprised to hear about this. My dad never told me, and he is the one in charge of this account. Looking through all the documents he stated back in July that he did not have my address or phone number to get in contact with me. Which is a lie because he has been to my house before and I have had the same number for over 6 years now. Plus all of my siblings, along with my grandfather (his dad) have all of my information. I firmly believe that if my cousin didn't sue him I would have had no idea about this and he would have kept my sons portion of the money. Should I get a lawyer? Or will my cousins lawyer make sure my son gets the amount he deserves? Any advice is welcome. I have no clue what I'm doing and I have absolutely no problem suing my dad.