Because we can trust Seymour to be an honest and upfront individual
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.Still EK herself mentioned ORS will not have incest, and as ORS is on steam now too, do you really think EK will take a chance like that? I don't think so.
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.