Falsification of Deed of Sale

I have an uncle who owns condo units. Some of it sold to buyers but titles were not transferred yet. He died more than a year ago. Then someone from city hall verified with us the authenticity of the deed of sale. They were signed by my uncle and notarized this year. My uncle is dead, so how can he sign them. I wonder who we can sue. There was a representative who also signed in the document as witness. He is the one ding the paper works. he got CAR, and has transferred the titles to the buyers. There are still some not transferred. Do we sue the representative and those who signed the document? Including the buyer and witnesses? Please advise.