The Gambit is a huge "Breeding" experiment ... and for breeding you usually exclude the ones with genetical disease like Seth. So excluding Seth is logical from a biological point of view.
True. But that's just us finding reasons after the fact.
When the details were given, it was never questioned. In gambitrevealone when it was explained, I don't think Elena & Yvette understood that without any explanation from Veronica. Especially Yvette, who had many questions, didn't question about Seth being excluded. Like "What about Seth, doesn't he fulfill the requirements of being Callista's son?"
If they knew Seth was child of Callista, or even if they only knew him as brother of MC, some questions should've come up atleast, even if those could be answered by your logic.
Also, that's only one aspect. Like Elena only mentions MC in relation to Callista, but equates Seth to Jake. Ofc that could be Elena being evil & only looking at Olivia's interests.
The overall tone of the whole game seems to differentiate Seth from MC whenever Callista is involved. After Olivia, Seth being unrelated to MC feels telegraphed kinda
Seems like it also explains why the newes Karlsson daughter is not eligable, because she has a disease too.
I hope she becomes eligible, or atleast available lol

Also, she was in cryostasis, basically dead. Leaving her out of everything, not just breeding experiments, was logical.
Actually, I think without those experiments on Seth, she might still be frozen & couldn't be unfrozen without dying from the disease

So thanks Seth (conditional on her having scenes

)