I had many sets of twins and trips, and I think the only looking more or less identical might be
the Adrian triplets. Perhaps they lack the final 5%, they all have green eyes which is no surprise as both parents have green eyes, too. I hate that eyebrows are quite random in my game, sometimes they even switch just from having a birthday, and they never ever reflect the parents' eyebrows. That said, in this game I feel the hair style is some times enough to make two sims look identical, especially when children.
Your experience with two sets identical twins in a rom makes me wonder if there might be a hidden gen controlling this? Genetics like that is definitely interesting :) Looking forward to see my Adrian triplets having kids, but as they are 8 years away from YA, it might not happen anytime soon...