Olivia is all but confirmed throughout the playthrough to be Elena's daughter by her behavior and meta answer
also the massive stat gains and lack of the default sis-boardmemeber stat for Elena. Instead Oliv-Elena has a Trust value. I guess there's still a faint chance Elena only thinks Olivia is her daughter while in actual fact she isn't. Elena herself seems very much convinced either way.
About Seth:
Yeah, I'm not sure Seth will just remain a pawn for all the other parties in the gambit. His position is totally passive so far - with little to no relevance even if you forego the good path of both Kane and Olivia.
Thus, I wouldn't be surprised if we see another twist with Seth where he's revealed to be either (or both)
- also Callista's son making Kane not the sole sperm donor option and all the politicking this could involve,
- or Alexander's (if so most likely only) son. But he wasn't up for the task due to his illness, which at the given point in time, may have been untreatable. (Or worse - lead to rumors of him being weak and having to undergo constant treatment. We can't have that in this dystopian corporate world)
Honestly, I'd like the second more. Though I'm not sure I have much to back it up.
It would make the figurative "the worse you treat your bro, the better it makes you look Olivia" a lot more literal and understandable. "Harhar! I'm willing to do evil even though I don't need to! Family means nothing to me! That means I'm willing to do anything to further my interests and this company! Harhar!" is not something I totally get behind.
It would also explain why Veronica has been constantly doing experiments on Seth (besides the obvious sadism and "research") - I suppose to try to get him presentable as a potential figurehead for the company. Something he wouldn't be capable of being in his current condition.
Cue more backstory on the original four why and how and when Callista left. Since Seth is the youngest, I think it is plausible to assume Olivia was either a guarantee of Elena's lasting loyalty (doubtful given how highly she speaks of Callista), a favor to Elena or even a request when Callista left/fled(?) (seems to me Olivia was some form of bargaining chip for Alexander to keep Elena in line).
Seth may have been another circumstance altogether after Callista was already "free" of the company's clutches and some other deal was struck to save / raise Seth. Maybe even one made by Alexander himself - to have his son become ready to realize his dream while in the care of the person he valued most.
Or, you know, Seth could literally be unimportant. Just another reason for Olivia to go along with the big paycheck until she discovers she's always been an evil b**** that gets off on others suffering.