Marriage

Traditionally, both Ninevarine and Moresain cultures are polygamous and patriarchal. Accumulating wives was a symbol of prestige and power for both. In the north, polygamy was also practiced by the Vendals but the gender roles were often reversed under the Vendals, with a queen having multiple consorts that competed for her affections. But neither has been openly practiced for a long while, save in the old Moresain holds, the cultural holdouts of Kolikis and Ghorvan, as well among the scant Vendalic tribes that still live on Talingarde.
The Axiomatic Church under Mitra preaches a certain principle of monogamy, of a transformative and romantic marriage that requires one love deeply (though not necessarily exclusively, there is allowances for courtly love outside of marriage). Thus, culturally, marriage is seen as this huge deal but it is also a romantic one. Divorce is difficult save in cases of true abuse of one partner over the other and a matter of great shame in any case. People don't tend to get married until their thirties (on the quickling timescale) because of how much focus is put on finding "the right one", and matchmaking and the like is a huge profession and cultural institution. This is one of the biggest roles the Church plays in day-to-day life, as almost every transition moment (birth, coming of age, engagement, marriage, children, death) has some liturgical involvement.
However, divorce is seen as a civil rather than religious matter among the Laithish and Guranese, and some privileges were given to them and the Kirk, a sort of split congregation within the church. Likewise, concubinage is a right of the nobility that is part of common law, and inheritance law in general is not part of canon proceedings but according instead to the traditions of your House/Clan. Thus, many dwarven clans for instance have "concubines" that are effectively additional wives.
There are also three other marriage traditions outside of the monogamy/polygamy divide above.
The elves have their own unusual tradition called "triunal" marriage. While elves practice Axiomatic monogamous marriage in Brecheliant, they sometimes take a "third" that is not merely a concubine but seen as an equal partner to the other two, but importantly is almost always from a shorter-lived kindred. Triunal marriage is an important diplomatic tool among the elves and came about during their travels through Astral Space as a way to promote cultural exchange. Almost all the Brecilien nobility thus are marriages between two elves of differing Houses, as well as a third (usually non-elvish aristocrat) to complete the trine. Triunal marriage is also the norm in the Commonwealth, with the added caveat of all marriages being arranged by the state. Two members of the same "caste" will be married, and then a third from another caste will be engaged to complete the trine.
The Rodkhora (along with related harfoot dominated cultures such as the Chimur, Sigilladi, and Marach) practice a tradition known as mesukoda. A mesukoda is a family unit, within which love is exchanged freely and all share the duties therein without prescribed hierarchy. While not all members of a mesukoda will necessarily be romantically involved (and romance is seen as an important component to such relationships, there's little sense of needing to breed for practical reasons), they raise all children born in common and generally speaking frown upon exclusivity within the mesukoda (with the exception of taboos against cosanguinity, which is why when a mesukoda forms it usually involves members from several different clans). Each culture has a slightly different take on the mesukoda, but it is the subject of much fascination (and confusion) by outsiders.
Finally, there is "alpha" family relations, primarily practiced by clans of beastfolk. In these arrangements, one or more "alphas" retain exclusive breeding privileges, resembling a polygamous marriage, yet the wider clan is also forbidden from breeding, usually with major sanctions. This means most beastfolk clans are highly familial, and much to the consternation of other kindred, the beastfolk do not have as strong taboos against incest. The kobolds of Rusante practiced this once but have more or less adapted to Axiomatic marriage in the centuries since their conversion. Additionally, the Sarpajit people under the thumb of the yuan-ti are often bound in such arrangements, with or without the infusion of ophidian blood. Interesting, this is also how goblins tend to naturally form relationships, though their breeding practices are even stranger than that and a mystery to most other beings.