A few considerations and thoughts to put out there, as I have read some wild speculations, some guesses, some good ones and also some things that put simply are not in the game.
This will not be comprehensive or exhaustive in any way and be based on me skimming through a bunch of comments.
Facts: Things that are clearly stated in the game or that are background elements to the game but may have remained not expressed.
Headcanon: my own speculations, interpretations, preferences, that are NOT to be considered Gospel even though I am involved in the making of this game. Any and all of this might be entirely subverted or negated in future developments. it IS true that some of my headcanons might become one day actual canon if it is a direction that the boss likes as well, but there is neither a guarantee nor an obligation in that direction.
Rabiah has had Nadia sometimes after MC became involved with Zahra. Was he still the father? it is unclear and remains not expressed nor insinuated. you can make up your own mind about this until the game tells you otherwise. for now, any opinion is valid.
The twins: Rabiah is not one of the twins.. the twins are her babies.. they are still nursing and will not appear on screen or anywhere else. It is not explicitly stated, but there is a heavy implication that Omar is the father and that Rabiah has settled for him several years ago and has become a bit more "boring" until moving back to Zeta.
Nadia has the biggest "you're not my dad" energy towards Omar and it is explicitly stated that she knows he is in fact not.
My personal take is that Rabiah probably was married to someone else before Omar, and that person would be the father. Rabiah has missed the babymaking train with MC when he went after her sister. I am personally not bothered either way, but I would think that MC would have been too busy enjoying his new wife and building his career to have time for the ex with the bad attitude.
MC and Zahra's finances: the compound they live in has been developed over time and is still susceptible to changes. this is to simplify the job for the boss and the narrative for everybody. At present, MC is rich and renowned. He is part of the 1%, so to speak. This is the result of 3 decades of work. Their approximate ages are: late forties to early fifties for him, around 40 for her. there's some discretion for you to give them whatever age works best in your mind.
MC had started going on explorations already, by the time he tied the knot with Zahra, and he might have gotten lucky a few times and struck gold. In other words, monetary concerns are background noise to the life of the MC, and even early on they were not hurting.
It should be noted that the spouses of Zetans who were in service of the city, specifically in the military, on missions and, one could argue, also Stalkers on official business, are awarded protection by the Palace.
So "if" Zahra had been financially unstable during the early years, as long as MC was doing jobs for the Queen of the day, she would have been provided for, granting some measure of stability.
Zahra's fertility over the last 18 years is an interesting topic. I would submit that they might just have decided that 2 girls was enough for them.. and that they took measures to ensure that things would stay that way. you may make up your own reasons or methods.
My personal take is that maybe the pregnancy with Ain was complicated and dangerous, and that Zahra, in agreement with MC, decided to use a fantasy version of the pill to stop it from happening again. maybe now she's reached an age where pregnancy is just extremely unlikely so she has stopped taking the pill.. maybe she's become barren in time.. maybe she will pop out a surprise baby. it is not particularly interesting to me, but there are dozens of ways or reasons for this to be perfectly justifiable.
likewise, maybe the multiple experiences in various Zones have left the MC able to procreate only in special situations, or maybe on command... or maybe the reason that they haven't had any more kids or that he hasn't inseminated every girl presently in the game, is that it's a game and if there's a pregnancy kink to explore, it probably should be done with active participation and choice in the matter on the part of the players.
these are all thoughts of right now, do not hold me to any of this.
The hit and miss realistic approach to historical impossibilities or scientific inaccuracies or technological implausibilities is due to the nature of the game, the limited resources when it comes to show things (there are only so many ways you can portray post-apocalyptic reality without getting lost in the weeds of what can't be, especially this limitation is true if we consider the existing library of 3D models and tools. none of us are actually 3D artists who draw objects from scratch).
What we do toss in is to reflect the movies and games and other forms of artistic expressions that are inspiration to the game.. and yes, sometimes people who have even only surface level knowledge of a specific topic will be pulled out of the narrative because of such an inconsistency.
Having been around horses a lot, I experience the same when I look at the charge of the Rohirrim at the siege of Helm's Deep. those horses should have rolled all over each other, on such a steep incline... and yet I am perfectly fine with Elves, Ents, Nazguls and so on.
A solid amount of handwaving is demanded to be able to enjoy this game. we try our best to make things somewhat believable and my personal interest goes into making this as coherent and logical as possible, but it will never work if you look too closely...
I believe that our best shot is to make believable characters have believable and meaningful relationships and reactions to one another (within the logic of occasionally going all porny for the sake of porn).
P.S. don't come at me for technical issues. I know less than you about them.