You're free to determine the level of blood "kinship" in the main family according to your tastes. Once you have started the game, you can reach the menu and edit the relationships from a dedicated feature. A couple specific relationships trigger dedicated dialogue options. (I don't remember which ones specifically, and it's mostly cosmetic, no dedicated plotlines, just for the dialogues to make sense).
In any way that matters, Zahra and Zaton are the parents of both girls, who consider themselves siblings of one another.
Whether by blood or for having raised them does not change the emotional and familial connections and roles.
They are the ones who have raised them.
I mean, you could go with the whole roomies thing but... Why?
Other than that, it is for now hard coded that Rabiah and Zahra are actual sisters (although we do not specify if by blood or, again, by having been raised by the same parents), Nadia is Rabiah's actual daughter, and she had her before she met/married Omar, with another undefined man.
What is established today is still subject to revision as the game development proceeds, should we deem it necessary (we try not to write ourselves in a corner though, so...).
Until it's finished, it remains a work in progress.
That said, you are free to fill the gaps with your imagination until we determine otherwise canonically.. if we decide to define things at all. Sometimes it's nice to leave things unanswered and open to interpretation.
I personally, as a player before being a Dev, appreciated the similar physical traits between Zahra and the girls, I see some resemblance between her and her sister Rabiah, and also between Rabiah and her daughter, so you can imagine how I have defined the relationships in my main playthrough. Then again, the web is full of pictures of dogs and dog owners that look alike and are obviously not related
