1) I don't really get the concept of relations in their family. So those two girls are step-daughters to MC and to his wife? I think in some of the dialogs the wife mentioned that. So those girls are adopted or something. They might be the Children of Zeta, born by some aspiring priestess. And then adopted. Am I right? I know there is a note in the library but it's kind of hard to get through.
So in that case they are not relatives by blood, and their relations aren't like those we all used to, but rather like those described in Robert Heinlein's 'Moon's a harsh mistress' or in the Expanse series (Holden's family, Drummer's warband).
Would be nice if the developer provided a clear definition to their relations. It's disturbing to think of an incest. Also the younger sister looks like 'mother', and the older one looks like 'father', it's confusing too. I doubt now that even 'sisters' are relatives by blood.
2) There seem to be some other misconception. What of their age? MC mentioned multiple times that he got married 30 years ago or so. That makes him at least 45 yo, and his wife too. They look too young. I understand there might be some rejuvenate techniques, but still. Also when we see that old master who will become lord of Decayed, the MC already has a beard, and he isn't married yet.
3) There are clearly a mistake with dates. In the 'Trainee' note we see the same date as in the 'The Long Walk'. It is clear that 'Trainee' should have at least Y437, like in the 'Reflections'.
These things are important since the game has such profound take in history. Any porn without history is boring.
As others noted, you get an option to set the relationship because they're not allowed to say they're direct children due to censorship in other sites. You thus end up having to go through this really annoying section where you set the specific relationship manually even though it's obvious they're supposed to be your direct daughters. Both look like Zahra, and Shani's lighter skin and blue eyes hearken to the MC.
Regarding ages..
I believe the marriage is actually only 20 years, as at one point the main character says he's held Zahra's hand for 20 years. The relationship might have started quite a bit before the marriage, but I get the impression it was a relatively quick courtship. Still haven't actually seen the full reason why he left Rabiah, but he certainly got his relationship with Zahra started out at full speed, especially if he just grabbed her head and used her mouth and throat after she coyly signaled she wanted him.
BTW, It's likely Zahra had full intentions towards him pretty early on, given how she went about quietly pursuing him after having to wait so long for her opening, especially as that first oral with her suggests she had been practicing behind the scenes in prep for him. This is one of the reasons why I expect the courtship didn't take that long. Disentangling from the Rabiah relationship and dealing with possible blowback from the sisyers' family might have been major factors, but Zahra clearly wanted to go there, already, and it's likely the MC didn't deliberate, either.
Assuming Zahra was 18 when she imply-offered to "finish" the MC when Rebiah did not, and assuming it was a two-year courtship, that would put her at about 40. I'm going to guess the MC had at least 5 years on her, so am assuming he's abt 45. That's assuming minimal age of 18 for adulthood however... See the next point, below.
Regarding the artwork and their ages..
I also have noted that the characters look younger than one would assume, given the necessary ages; this is especially true of the girls who are noted as being only recently of adult age, such as younger Zahra (in the flashbacks/stories told to Nadia) and Ain. Notice that when the characters say someone is "of adult age", that's not a specific, spoken number.
Now, I'm not saying anything was taken into account here in the game.... But the Pre-industrial/traditional standards of many societies around the world peg the age of adulthood about 5 years younger than nowadays, and the artwork strongly suggests that standard, tho it's always possible to write that off for stylistic reasons. For a certainty, officially every adult is 18 or higher, in this game.