After a long time of playing Sims generation upon generation, I found that each feature in a child sim is random from either their mom or dad. What you had with the daughter looking like dad and son looking like mom's eye- and hair color-wise, should be a coincidence.
In twins, I think they're either identical, or they get (mostly) opposing features from their parents. So in twins, they either look the exact same or the complete opposite. ?
Because of this, non-identical twins often look more like their other siblings, than like their twin.
Here's an example from my current played sims:
Left to right: Monty (dad), Kaylin (firstborn), Emma & Caroline (twins), Hallie (mom).
Notice how Emma and Caroline have opposing hair colors, eye color, skin tone and so on.
Caroline looks more like her older sister Kaylin, than like her twin Emma.
(open in new tab to have a closer view)
https://i.imgur.com/wHEzwEt.png
The more you know \o/
