Update: This has nothing to do with relationships between Sims in the game. There will still be Brothers, Cousins, Grandfathers etc. (See the Full List). The thing that we won’t have is the Family Tree User Interface that shows all the Sims that are currently living (and who are dead) who belong and/or are a part of that family.
Pic from The Sims 3 Forum
Also, yes family trees are really not in the (base) game as far as I could see.
— Rens Verhoeven (@renssies) August 14, 2014
— SimTimes Team (@SimTimes) August 14, 2014