He could simply not know about it? He fucked her mom, broke it off and she didn't tell him about becoming pregnant since she doesn't want him to associate with her children.
Seymour's not her dad but everything hints that he
did have a major thing for her mom, hence the obsession with Lena, a do-over he can control. Foreshadowing led to some people putting that likelihood together by Chapter 6 or so. All but confirmed that's likely the way this is going as of some text in Chapter 11. This next part is more of a guess, but Lena's dad is more likely to be the bum "cuz red string" (that deus/diabolus ex writing crutch used in the game to explain how nearly every introduced character has some extra "surprise" connection to the MCs). Seymour knew the bum years ago before his life was destroyed (likely by Seymour) and Lena's description of her real father in Chapter 9 makes him sound like more of a deadbeat (he left when she was a toddler). Eva also wants the game on Steam eventually & wouldn't risk father daughter sex scenes in this one the way she did in GGGB. Daddy making her stick devices up her twat would likely be a no-go there.
Also: "I have no real memory of him, aside from what I've seen in pictures and what my mother told me." Seymour is rich and powerful enough to have some famous notoriety. If Seymour was her father, she'd know. Where-as the bum is this disheveled mess now, likely unrecognizable. And we only know his first name as of now - if it's even his real name. I do hope it's
not the bum personally, since I'm not a fan of these "cuz red string" connections that make the world feel small. But that's been my guess for a while as Eva's clearly a fan of these coincidental pairings in this game. If it does happen, my initial guess at the unveiling will likely be: Emergency trip to the hospital for mom. Bum shows up, visiting her?!? Likely learns about Seymour's relationship with Mom at that time too.
btw, I don't consider Seymour having a thing for Lena's mom a "cuz red string". His obsession with Lena clearly shows a motivation to search her out, which is very different than a crazy coincidence. Basically fine with that plot turn if that's what it ends up being. Note: I wouldn't consider Bum = Dad excessively "Red String" either if he actively approached her either knowing or because she seemed familiar, looking like his ex wife. Though the timing of running into her now is still excessively convenient without use of an additional catalyst.
