@JonaO703 I'm also not seeing aunts, uncles or cousins and it's the same even if they live together, which really doesn't make sense. The relationships do show in the individual Sim profiles though, but not their family trees.
So for example, Malin has her parents on her family tree along with her brother and sister who both live in other households, but not the Aunt who lives in the same house as her. They were all created together as well using Play with Genetics. Malin's niece and nephews aren't there either from her siblings households. I checked the other family households too and it's the same.