It is not(!) proven in the game that Seymour isn´t Lena´s father! Seymour might never have been told by Lena´s mom that he is the one and he tells it to Lena the way he is aware of, that he has no children, but the way he treats Lena on most path variants hint strongly that he suspects she might be his daughter.
Come on, Seymour is very much simping after her, he could recruit established supermodels without problem, but he chooses Lena and can be quite lavish in his attention to her. It might have started as a chance encounter, but Seymour seems to suspect now Lena could be his child. And considering how bad her mother treats Lena most of the time, hardly as part of the family, is another hint that she is probably Seymour´s daughter.
Remember that Syemour got rejected for dubious reasons by a woman? Lena´s mom might have been involved with Seymour, cheating on her husband and when she got pregnant had to pull the plug. She would clearly know that Lena is Seymour´s daughter, a part of her life she wants gone. Lena is treated by her mother like an ATM and not much more. She is on the very outside of the family, her younger brother, a waste of space, instead get´s treated with lavish love by their Mom.