I think is 99% impossible that Seymour is Lena's "real" dad. Lena's "real" dad is not an unknown figure for her, it's not like her mother doesn't know who he is or never talked about him.As I wrote in an earlier post, Seymour might not provenly know it, just suspecting that Lena is his daughter. That something into this direction seems to be in the cards is that Seymour never touched Lena sexually, but is very, very much simping over and supporting her. As mentioned, Seymour with his money and connections could easily book renowned supermodels for his photo hobby, zero problem.
But instead he takes in a newbie, if slightly rising, model with Lena. I speculate that Seymour either learned of his possible offspring before meeting her first or after their first meeting when he decided to look further into Lena´s life.
I do not base this solely on speculation, but also the fact that there is no really sexual contact between Lena and Seymour, two of the major characters in game so far. That goes against EK´s typical MO. This far in (ch.11) normally the couples which could happen had at least some "naughty fun" together or are close to it.
Besides, unlike the bigot fuckers at Patreon, Steam has few problems with incest in games.
Python:
l "Well, technically he's not my {i}real{/i} dad, but it never made any difference to me."
$ fian = "n"
i "I didn't know you had a stepfather."
l "I've never used that term. As I said, to me he's always been just \"Dad\"."
l "My real father left when I was two. I have no real memories of him aside from what I've seen in pictures and what my mother told me."
l "But Dad has always been there since I can remember. He got together with my mom shortly after, helped her take care of me and my brother."