Even a surprise is extrmely unlikely. We know, they knew, on the pier, what the sex of the baby was. that's 12 weeks minimim...literally minimum (though could be 10 weeks but only roughly 70% chance). I'm putting it at the very earliest possible time (on my timeline) given we know Callista was a renowned scientist etc and the tech we are shown. I'm pretty sure she could find out at the earliest possible opportunity.
So, at that time-frame, it would be impossible for Callista to
not know she had twins. So I think we can 100% rule that out completely. It also can't be one of Alexander's sisters as, as you say, too convenient, though it would make things in the facility lol (and if was, did Alexander know? Grrrhmmm...fun theory but not really seeing it).
As for likely being dead? I'm not sure on that one myself. We've literally heard nothing about the woman at all. Nadda, nuffin, zilch. Just like at least one (possibly) two other ladies mother(s). Intentionally not spoken of ny them? Intentionally not mentioned by Tess to keep things vague/people guessing? Really dead? Just, not involved currently Dind no reason to be (thus no reason for them to be mentioned, remember, we're only on the eigth day after all, sixth day at the facility)...