how do you make family tree? property with heirs

[Isaac El]

I have been working on a property for a few months and they agreed to sell it to me, was soooooo excited.  Then attorney calls me and says "they cant sell it to you, because there are two other siblings that passed away, so their shares go to their children" in short I have to find those other 8 people.  The person I was talking to says they have not talked to that side of the family for over 30 years, plus other family issues she disclosed to me.  One estate attorney told me I need to make a family tree, any advice? Would hate to lose this deal.