Yes, I know, that's why I said it could be a longshot, but then you have to wonder what else would be the purpose of this bonus storyline? At the end of the day, this is an adult game and sexy fun times are kinda the main point, so it's hard to imagine this being just an innocuous storyline that won't lead to anything sexual.
We're also only at playable day 11 of 30 and a lot can still happen. Sophia's progressed quite a lot in such a short space of time and Ellie could change too, especially if we get some more time jumps. Plus, Dylan noted that what she said after the hug didn't seem laced with the usual animosity, so if he can continue being a good brother to Ellie and Sophia continues doing things with both of them, I could see those two worlds collide and it leads to Ellie and Dylan doing things with each other.
She may not go from bitchy sister all the way to being a softy sister, but she might at least get comfortable with a casual thing with Dylan where they can use each other to satisfy their needs.
Maybe, but I'm hoping to see Sophia get involved somehow. Perhaps she sees them kiss, waits for them to finish, then she approaches them and they all go into the pool together where Ellie gets a bit handsy and it builds more tesion between them.