And therein lies the problem. The original Hargreeves siblings have no doppelgangers in the Sparrow timeline. This means their presence is a paradox, and that paradox creates the kugelblitz that triggers The Umbrella Academy season 3's apocalypse. As Elliot Page's Viktor soon discovers, these doppelgangers are missing because Harlan Cooper killed the Umbrella mothers (plus a bunch of other women from the original 43 births that happened spontaneously across the world) before the babies were born. Why would sweet, innocent Harlan carry out this maternal massacre - especially those whose children weren't in the Umbrella Academy - and how is such a feat actually possible?
The Umbrella Academy's mother deaths are sealed in 1963, when Viktor accidentally imbues Harlan with powers while saving the boy's life. Attempts to remove them are unsuccessful, and Harlan spends the next 50 years struggling to contain the White Violin's awesome power. As proven by the bunny and bully incidents, emotional situations trip Harlan into losing control, unleashing a destructive vibration similar to Viktor's. Because Sir Reginald suppressed Viktor's powers for so long, old-man Harlan is way more knowledgeable about their gift, and explains they can "feel" the vibration of other super-powered kids. So, when Sissy passes away on October 1, 1989 - the day the 43 children are supposed to be born - the grief-stricken Harlan senses Viktor. Psychically reaching out, he finds Viktor's mother shortly before giving birth instead. His grief combined with her fear triggers Harlan's biggest outburst yet. Completely unintentionally, his vibrations cause brain hemorrhages in Viktor's mother and 26 others.
It makes sense that during his darkest moment, Harlan reached out to Viktor - the only person he loved besides Sissy. It also makes sense that upon feeling the overwhelming fear inside Viktor's mother, Harlan freaked out and accidentally killed her, just like the pet rabbit who bit him in the flashback montage. But how does The Umbrella Academy season 3 explain the grand total of 27 dead bodies in Harlan Cooper's wake?
The science behind The Umbrella Academy's mother deaths is predictably murky, and deliberately vague. Think of the 43 children and their respective mothers as a network - each connected to the other through vibrations. Harlan and Viktor's power is manipulating vibrations, and this includes the resonance from their super-powered siblings. When Harlan reached out to Viktor's mother, he automatically connected to every mother, creating a chain reaction from one to the next. The mothers of the six Umbrella children Harlan met in 1963 probably died first because their specific vibrations were familiar to him. This would explain why all of the original Hargreeves sibling mothers perished. From there, the order might've been totally random, or based on physical proximity to Sissy's California hospital bed. Harlan eventually cut the connection, but had he not regained control, all 43 The Umbrella Academy mothers would've died.