My Sims have a twin (a boy & a girl). Totally different in terms of features & the eye colour, but both of them have the same hair colour & skin tones.
The sims in my profile picture are twins. So they have the same eye color, but different hair color and facial features.
But I have also had multiples that look very much alike, so I guess it depends.
They can be identical or fraternal. One of my current households has a pair of identical twin girls. I was really glad they turned out to be identical, since their names (Demeter and Ceres) were taken from the Greek and Roman names for the same goddess.
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But I have also had multiples that look very much alike, so I guess it depends.