Hart's Farm has two moieties, the farmborn and the outsiders. Their arrangement makes sense but does not duplicate what other moiety cultures are doing. 1) The farmborn are primarily members of the same family descended from the founders. Farmborn do not mate with each other. A farmborn baby has one farmborn parent and one outsider parent. 2) The outsiders are primarily people coming in from elsewhere, not related to that family. They can mate with a farmborn or another outsider. 3) Someone born off the farm, but known to be related to the founding family, is counted as farmborn. 4) Someone born on the farm, but with two outsider parents, is technically an outsider and thus allowed to mate with a farmborn. But they usually don't because they grow up with the farmborn, like siblings, so most of them prefer to mate with outsiders instead. 5) When parentage is partly or wholly unknown, people make their best guess. A baby conceived on the farm is usually considered farmborn and one conceived elsewhere is usually considered outsider.
So it's kind of like ... a moiety and a half, with some blurry edges. The one firm rule is against mating between two farmborns. Another important rule is that if something doesn't cause a problem, it should be accepted and not fussed over. They are far more concerned with maintaining a healthy, comfortable community than with enforcing arbitrary expectations on other people's lives.
Marriage is allowed but it's a trivial minority of relationships on the farm. Hrafn and Gróa are married but I think they're the only monogamous relationship in canon so far. Most people just take lovers, short or long term, from whomever is eligible and willing. That tends to create a web of favorite partners who often hitch up in twos or threes, and it shows in the parentage of the children. Woven around and through this are other relationships, some of them quite strong and permanent -- best friends, romantic but nonsexual connections, erotic but noncopulatory connections, and so forth. There are a few people who are less connected but most characters have a dense mesh of relationships, spanning romantic and platonic, with many other individuals. The primary bond is the community, rather than a nuclear family.