I have been bringing this up in every thread that is relevant. She is the same Sim. The game does NOT reset the sim in CAS. All these people talking about mothers becoming sisters or roommates need to make sure they are not using Sims from the gallery, editing them, and then placing them in the same world that already has that Sim. They are the same Sim. Always, always, always make a twin.
As for making a twin...it should be the exact same facial features. Unless this is related to the aging glitch in the male features where they get smaller eyes and a larger chin. All the twins I make look the same. I will go and play a bit more.
