When American Horror Story: Apocalypse delved deeper into Michael Langdon's past in "Return to Murder House," it took a step back in time to the show's very first season. Michael's entire backstory is dependent upon the plot of Murder House, but it's been complicated somewhat by his position as the Antichrist — and the fact that his biological dad is kind of dating his biological sister. This Michael Langdon family tree breakdown will make sense of all the AHS family dynamics, even if it relies on a willing suspension of disbelief now and then.
Everything began with the matriarch of the Langdon family, Constance. She welcomed four children, all of whom met their demise relatively young: Beauregard, who was killed on Constance's orders after a lifetime of abuse at her hands; Adelaide, who was hit by a car; Rose, whose cause of death is unknown; and Tate, who was shot by police after murdering countless students during a high school shooting. Per the rules of the Murder House, those who died within its walls became ghosts. Tate was trapped in the house after his death, where he became close with another ghost named Nora Montgomery. That relationship set a plan in motion that resulted in the birth of Michael Langdon.
Ghosts in AHS become stuck in repetitive loops, often obsessed with the same goals they had in life. Nora's child was abducted and murdered when she was alive, so as a ghost she became preoccupied with the idea of having a baby. Tate wanted to help her, and he saw an opportunity when Vivien Harmon moved into the house with her husband Ben and daughter Violet.
Tate disguised himself in a rubber suit that belonged to a previous resident and raped Vivien; she initially believed that she was having sex with her husband Ben until events later in the season made it clear that that was not what happened. Vivien became pregnant with twins, which was where the situation became even more complicated — as if it wasn't complicated enough already.
While Tate was falling for Violet (who was completely unaware of his many misdeeds), Vivien received shocking news about her twins. The boys had different fathers: one was Ben's completely human son, while the other was Tate's half-ghost baby. And that ghost baby proved to be trouble right from the start. Not only did he cause Vivien pain (and a craving for brains), he was prophesied by psychic Billie Dean Howard to be the Antichrist. She claimed he would bring about the end of days. He was "the essence of evil, a perversion of the Immaculate Conception."
Vivien died in childbirth and only one of her sons survived. That was Michael Langdon, who ultimately ended up in the care of his paternal grandmother Constance. Michael is technically Vivien and Tate's child, though there is still some question about who exactly is responsible for his conception. In "Return to Murder House," Tate wholly rejected Michael as his son. His rape of Vivien brought Michael into the world, but Tate was apparently being used as a tool by Satan himself. Supernatural genetics wade into some pretty confusing territory.
"The Holy Ghost merely whispered in the Virgin Mary's ear and she begat the son of God," Billie Dean said, way back in Season 1. "If the Devil's going to use a human womb for his spawn, he's going to want a little more bang for his buck." Vivien echoed that idea in Season 8, claiming that neither Ben nor Tate was Michael's father. His dad was actually "the source of darkness," i.e. Lucifer.
In conclusion, as illustrated by the very professional graph above: Vivien is most definitely Michael's mother, but his dad appears to be a combination of ghostly assault and Satanic intervention. And his Ghost Dad is in love with his Ghost Sister, even though Ghost Dad raped his Previously Human But Now Also A Ghost Mom. (Don't Let Tate Off The Hook 2k18.)
That will definitely not be a fun Thanksgiving dinner.