While I don't think there is any specific rule in the game's universe that a stalker cannot fuck his successor (regardless of the blood relations, since that can be changed by player), I got the impression that a stalker getting in a relationship with his/her successor is perhaps frowned upon, within the kingdom, sort of like how incest is perceived in our modern day society.
When our Mc rubs his dick on Shani's ass, iirc he says something along the lines of "I really shouldn't be doing this" feels in a lot of ways MC's morality basically screaming at him to stop but also what people in general perceive as okay or not in his world (even though given incest is totally okay in this world, so I guess their morality is pretty skewed, heh).
This screaming "TABOOOOOO" factor is just what makes Shani even more attractive/compelling in my eyes though.
Breakthrough with the whole body-to-body relationship with out elder daughter will really feel like obtaining that forbidden fruit.
Yup, MC can also fuck Ain (youngest) to "punish" her in their basement in front of Zahra (wife). First smack her bare bottom couple of times, then choice comes to fuck her.
I think when Zahra sees that she says "Husband, the point here is to "punish" our little daughter, not to give her pleasure." (or something along those lines)