A threesome from Sophia / Elli / Dylan somehow looks as if this route was added later from the perspective of Sophia told. But that's just my very personal opinion.I too feel like there would be too many variables or issues between siblings to attempt such a taboo trinity, yet that doesn't make me want to see it as a viable path any less. Story-wise, I wouldn't propose Sophia intentionally work towards going down a path that leads specifically in a 3some. I would be interested in a possible storyline where Sophia considers ways on improving her children's relationship and budding sibling rivalry over her attention and ultimately erotic affections.
She then decides its in their best interest to create events/outings where they would both have chances to interact (Dylan and Ellie exclusively without distractions from their 'friends/lovers' though maybe occasionally bring along Auntie Patricia) and with her present. In this way they could put their differences aside, actually begin to bond and get comfortable being around each other, building chemistry and trust. Ultimately this could end up leading to a weekend getaway just the 3 or 4 of them culminating in several opportunities for them to take a series of steps closer to real physical intimacy that may or may not lead to actual foreplay and variations of sex.
Again, while I expect resistance from Sophia intentionally interested in that specific kind of closeness, I could see Ellie and Dylan bond over their shared physical attraction to Sophia, helped along and even encouraged in most cases by Patricia. Fingers crossed this actually becomes somewhat of an actual path for the 3 or 4 of them (if Patricia is included).
On the other hand, I consider a threesome Sophia / Patricia / Dylan possible and if the perversion of the two women escalates and they repeatedly challenge each other, it is even inevitable. The only difference is that things won't go the way he imagined for Dylan: The two women use him as a sex toy.