One of the things I've spent some time doing for the Dawnlands is figuring out what kinship system the four main cultures - the Kadiz, the Hill People, the Kaddish and the Dwer - use. It turns out that there are only eight basic kinship systems in the world, and every known culture uses one variety or another, with most falling into six classic patterns.
Modern Westerners use the "Eskimo" kinship system, which emphasises the nuclear family. However, this is unsuitable for the sprawling clans and gentes of the Dawnlands, so I decided to use two common patterns, one more recognisable, one less so.
The more recognisable kinship system is the "Sudanese" system, which Anglo-Saxons, Romans, the Chinese and evidently the Sudanese use. It's the most common type in the world, and it's a "pure" descriptive system, where every person is described according to their relation to the describing individual. This differs from the system most of us will be familiar with in that things like one's mother's brothers and one's fathers brothers are given distinct names, often overlaid even further with distinction by age, so that that "oldest brother of my father" and "youngest father of my brother" are distinct. Sudanese kinship systems evidently occur frequently in complex, stratified societies, perfect for the Kaddish and the Dwer.
The less recognisable one is the one used by the Kadiz and the Hill People, "Iroquois" kinship.
This looks exceedingly complex at first pass, but I actually chose it because it's comparatively simple and it makes sense within the cultures of the plains. The eponymous exemplar of Iroquois society is matrilineal, but the Kadiz and Hill People are patrilineal (kinship passes through men).
The basics of it are simple, you are related closely to your father, your mother, your father's brothers, and your mother's sisters, and their children (including children your father has had by other wives, if any, but not including any children your mother had by previous husbands). The relationship is close enough that reproducing with any of them would be considered incestuous. On a side note, if a woman declares herself to be male by getting a name tattoo, she is treated as a male for all purposes, though such women rarely have children. If they do (which is shocking by plains morality), they are relatives. Also, any catamites / eromenoi your father or uncles have are relatives. For hard numbers, this is typically between 20-40 people, since nomad families tend to be large.
On the other hand, your mother's brothers and your father's sisters and their children are not considered close enough to fall within incest taboos (they are "cross cousins" in anthropological jargon). While still related, they are distant relations, comparable to second cousins in our own society.
Your sept (your Kadiz character's "middle" name) is your paternal grandfather's, who founded the "sept" to which your father and his brothers belong. Your clan is an agnatic descent group from some male ancestor.
Relatives are a ready source of allies, warriors, friends and expertise who are customarily obligated to help you, unless you have been declared a bandit, in which case they will probably help you anyhow and pretend not to have.
The Kadiz and Hill People are constantly raiding one another. Around a 1/3rd of all women are abducted from external groups, and their fathers, brothers etc. may be trying to kill you or at least may raid you with the intent of abducting your female relatives (or you may be doing the same to you). This explains why doing so does not constitute incest, since the same groups tend to raid one another over and over again due to geographic proximity.
This hashes out into the following two new uses for the Culture (Own) Skill:
Who Will Back Me Up In a Fight?
A character may test Culture (Own) to determine how many of their male relatives and friends are available for violent purposes. This does not mean they are on hand at the present moment, simply that they are old enough, skilled enough, physically well enough, and closely related enough that they will be of some use in violent struggle.
A successful check lets the PC know an exact number, along with a clear idea of how they can be reached. The clear idea may not necessarily be convenient or even possible (if, say a PC is imprisoned). If the referee does not have a specific family tree worked out, it may be convenient to roll 3d10 for the number of warriors who can be summoned. This number may be increased if a PC is part of a particularly fecund or prominent sept, or reduced as appropriate if circumstances like a previous disaster or depopulation have reduced the family.
A critical success allows the PC to know an exact number, along with a workable plan on how they can be reached as rapidly as possible. This may still be agonisingly slow under the circumstances, but the plan has a good, though not inevitable, chance of succeeding. The PC can roll 5d10, as minor relations are encouraged by closer ones, and a portion of the broader sept and clan come to the rescue. Alternately, if the PC has established a common ancestor with a group who normally would not consider themselves relatives, a critical success allows him to call on them as allies instead. 3d10 will arrive to support him, and they may be much more accessible than distant relatives of closer relation.
A failed check means the PC miscalculates, perhaps forgetting whether certain relatives have come of age, or are injured, or off elsewhere. They do not receive any useful information from the referee about how they can be contacted, and must formulate a plan on their own.
A critical failure means that the PC miscalculates the capability of his family to support him, and may in fact arrive at what he thinks is a workable plan that is spectacularly incorrect. Examples include sending a messenger to the wrong seasonal pasturage, forgetting to equip the messenger with tokens of safe passage or a description of one's name tattoo, interrupting an important clan ritual with one's complaints, etc. The net result is that either no one comes, or they are significantly delayed beyond the point of usefulness.
Who Am I Feuding With / Who Am I in Debt To?
When one member of a sept enters into a feud or incurs a debt which they cannot repay, it is the responsibility of the entire sept to ensure that the feud is discharged honourably or the debt paid off. A successful Culture (Own) test allows one to keep accurate track of these matters, so that one is not surprised by the debt-owners or feuding enemies. Failure simply means that one loses track of it all.