So I have a world with a moiety system (basically just means division of a people into two groups), and I want kids to only inherit their moiety from their mother - b/c I have it set up so that only opposite-moiety people can romance each other, so if they inherited from both parents, they'd just get both groups and the system wouldn't work anymore

Wait never mind I figured it out - I can have separate castes like morning-female, morning-male, evening-female, evening-male and only have the morning-female and evening-female castes inheritable

wait no that doesn't work either b/c the second generation, you'd have some dudes with the morning-female and evening-female castes and they'd pass that down to their kids too when they're not supposed to

Also just realized i put this in the wrong forum - i meant to ask this in storyprogression, sorry about that

Post moved. And sorry, maybe I don't understand the question fully but it sounds to me like you only need one inheritable caste, not two or four. A sim is either a member of Group A (they are in the caste) or Group B (they are not in the caste). So a Town Option would be that no one can romance/breed with the caste members, but a Caste Option on is that members can only romance/breed with caste members. Caste Options will always trump Town Options.

Too simple?

I think the idea is that all couples should consist of one sim from Caste A and one from Caste B. Therefore all children will have one parent in each caste, and the OP wants settings to force each child to inherit exactly one of these castes, with a more or less equal number of children landing in each one.

I don't think this is possible with how SP works. There's no way to say a caste is inherited if and only if the parent who would pass on this caste meets certain criteria.