I admit that you know better than me the conversations and the storyline regarding Ellie, I missed that part because Sophia went to the shower and was heard by Dylan instead of being caught by Ellie in the living room and in my game, she didn't shower with Ellie the first time, the only thing that I can think of is by one of the images posted here, I don't remember if it was by you about a conversation about masturbation.
You're right, they're not that close enough, for now, but maybe down the line as you say there will be a consequence or something about it, besides, she was seen Dylan masturbating several times already so I guess it's just a matter of time when Dylan talks about it with Sophia and what he heard. The walls of privacy and inhibitions are falling down around Sophia with both her children and that's what matters anyway haha.