He could discover an old magazine having nude photos of his mother when she was young and kept it a secret rom everyone becaus she was ashamed about it so that was her little secret hidden from everyone and Dylan discovered that and kept also a secret in fact that could be his way of corrupting his mother later on, and how it could kill any trust she had to Dylan if she doesn't know about it
If you check the earlier versions of the game you will see that Dylan's milf mags do not have nude models in them.
Dylan already has pictures of his mother in her underwear and in bikini's.
Dylan is also ready corrupting Sophia so why does he need another one?
If Sophia did pose for nude pictures,it would of been for Liam,and we have indication that she ever did so no reason to be ashamed.
I am sure that if Sophia found Dylan looking at nude pictures of Sophia I am sure she would be soooooooooooo happy(not)