Yes, since pictures cannot be exchanged that is surely proof that Lena knows her biological Dad, sure!Her mother treats Lena like a servant and money sender, nothing else. Her stepdad is the ONLY one in her family treating her decently.
I would be less than surprised if Lena´s Mom did not exchange the pics of Seymour for some random guy, because she knows the moment Lena learns that Seymour is her Dad, the money Lena sends will go away forever.
Just remember that Seymour, no matter his pros and cons, treats Lena vastly better than her family ever did even on the worst playthroughs.
So, you think Lena's mom put up a deep cover operation to hide Seymour from Lena. Because she could foresee the future where her husband was gonna get cancer, her son was going to be a junkie and she was going to have also health issues and would need to depend on her minimum wage daughter who was going to move to the same town where Seymour lives. I mean, Seymour is a millionaire and she could have gotten child support from him, but she chose your cover up theory.Lena gets treated like a doormat and money donator by her Mom currently and some dialogue hints strongly that this has been the case for a long time. Now just think about what would happen, if Lena learns that her Mom beside treating her like an unloved add-on, also lied about who her real Dad is?
Most people would be very pissed off and Lena hardly an exception. Many would cut off contact.
Also, she hided Seymour's identity in order to avoid Lena getting mad when she discovered her mom lied about her real father... If you want the incest content so bad, just make it your headcanon. Maybe someone does an incest mod at some point.