Sophia would actually have a lot to regret on the Ellie route! But since L&P obviously feels the same way as you, that lesbian sex (even with your own daughter) is neither adultery nor taboo-breaking, if you only play Sophia on the purely lesbian route, Sophia can go into this event carefree.Assuming that you play those scenes. I think what threw me is that there's nothing on the Ellie route for Sophia to regret.
Didn't particularly occur to me that the teasers might refer to events on the Dylan route for an Ellie event, but that is probably my mistake, since Dylan does have a way of sneaking into events.
But this also means that you miss a lot of context during your playthrough.
And Dylan doesn't sneak in, he was there from the start, even if you did your best to ignore him. (I'm intentionally misunderstanding you here!)