i'm hoping to make a town that uses a moiety system, where only people of opposite moieties can date/have sex, but for this to work, the child needs to inherit their moiety from only one parent, instead of inheriting both, so is it at all possible to mark a caste as only inheritable through the parent that gives birth to the kid?

What exactly do you mean with moiety system, what do you want the new born to inherit?

it's just the technical term for the division of a society into two inherited groups

like, i want half the people to be marked with caste a and half the people to be marked with caste b, (separate from the male/female split, ofc), and caste a can only date members of caste b and vice versa, they can't date within their group

but that only works if the kids only inherit the caste of one of their parents, not both of them

What I mean is you define something in a caste like traits f.i.?

"The "Caste: Inherited" option specifies that when a baby is born and one of the parents has this caste, the baby inherits this caste"
I understand it that never give castes to male sims with option inherit enabled.

So I don't think this is easily possible, maybe only if you apply the caste manually to the mother, because automatic needs more clear definitions
- the mother must has this caste, so everything set is fitting for female?
so a male baby born cannot get it? I really don't know

I hope for igazor or Chain_Reaction to clarify

Like mother has (and with automatic other females too)
Caste: Inherited - True
Gender: female
Caste: Trait Require: - choose which
Sim: Disallow Trait - choose the opposites

Sorry if I'm oversimplifying this, but what would be the downside of manually assigning either Caste A or Caste B to each existing sim in town, and doing so whenever a baby is born or a new sim enters the world? You would have to constantly check on this with an automatic structure anyway to allow for newly arriving sims who have no mother on their family tree.

yeah, that could work - i was kinda hoping to, after the initial time investment, just let it run, but it could be feasible to switch households to apply the castes every time i get a 'baby was born' notification, i suppose. the only issue would probably be figuring out which sim was the pregnant one if I decide to allow for same-sex pregnancies.

You don't have to switch active households to apply a manual caste to a sim.

(hint: it's the one running to the bathroom to throw up every morning, with no evidence of a burned/spoiled meal having been served -- just kidding) There's several ways to see which sims in your world are pregnant. One is to use a filter on the MC Focus command (on City Hall or an in-game computer) and not only will you see who the preggos are in a list, but clicking on any of them will take you by way of the game camera right to them. Even if the sims themselves don't yet know they are pregnant.