Not sure of the episode title, but Discovery (2nd season) covered the possiblity that Michael was to die as a young child before the RED ANGEL interveined. I personally took this to be what occurred in the prime timeline. And why Spock never spoke of her.
Or she was never his foster sister because there was never an attack, her parents never died and raised her themselves. Maybe she still ended up in Starfleet serving with Georgiou in that other universe and the ship was destroyed or she ended up on Discovery and it was lost or caught in time, etc., The original timeline could have been altered in any number of ways. On the show Enterprise, Daniels tells Archer that there was no record of the Xindi War. There probably was not originally a Klingon War either or the other events we have seen in Discovery. There are tiny differences even from Season 1 to 2 of the show Enterprise — Trip had a brother and a nephew and parents he talked about in the first season; by Season 3 he had “one sister” and after her death he didn’t have a reason to visit Earth in the episode “Home.” No mention of his parents or the brother or nephew. The books offer an explanation for some of that but maybe the characters in the series shifted to another timeline where their fundamental family relationships were altered.