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
Crinrict, SimTimes & @Renssies confirmed that during The Sims 4 Presentation in Germany, SimGuruLyndsay told them that there are NO Family Trees in The Sims 4! Kinda a let down, don’t you think? :/
Also, yes family trees are really not in the (base) game as far as I could see.
— Rens Verhoeven (@renssies) August 14, 2014
(Just to note @SimGuruLyndsay simply answered no to the family tree question somebody else asked) — Rens Verhoeven (@renssies) August 14, 2014
@SimsVIP We had a presentation today. The answer for this question from @SimGuruLyndsay: no family trees.
— SimTimes Team (@SimTimes) August 14, 2014