If you put it this way, Ashe is destined to be Delilah's partner. (In some routes at least...)
A Fiona and Delilah threesome would be fun, maybe with siren Ashe having a strong say in who gets positioned where as I do like the idea of Ashe railing Fiona as she fucks her sister.
Agreed that Delilah and Ashe will eventually hook up and a solo scene with Delilah would happen first.
Now that we've already been fucked in the pussy by Fiona, if anything were to happen with the two of the sisters together, I think it's more likely that Ashe would be in between them with Fiona fucking Ashe's pussy again
while Ashe fucks Delilah. It would be having the two sisters sharing in a sexual experience together that could later (
same scene, after a while or another scene more likely) lead to the two of them having sex directly.
I don't think even a strongly dom Ashe would initiate that for them without the two of them succumbing to their desires on their own. It would be a group experience that opens them up to the idea.
The game doesn't have incest listed as a tag but we kind of already kind of had incest with the scene featuring Ashe's sister before and we seem to be heading that way with other character relationships as well so a Delilah & Fiona coupling is a possibility at least.
That is, of course, if the dev hasn't said explicitly otherwise and I missed it somehow. Haven't read all 238 pages of this thread yet.
Now, I don't really see this happening, but I would be here for it if the dev wanted to explore that kind of connection between the sisters. Ashe would definitely need to be the bridge between them, one way or another.