Hey, I'm a newbie to both this forum, and to Glorantha in general, but I've been reading up on the guide and various parts of the Stafford Library (the ones I've been able to get my hands on). My interest comes mainly from the anthropological side of things, since I'm working on my master's thesis in social anthropology. I just wanted to add, somewhat carefully (as there is a lot in here I haven't been able to read up on), that most oral-based societies across the world tend to be more flexible in their kinship structures than what the "official" versions often make it out to be. The way I tend to interpret the lore on laws and customs that are found in the background books, is that they are "ideals" rather than an all-encompassing description of facts on the ground, as it were.
Examples from real life that come to mind are the historical Germanic tendencies to "invent" mythological ancestries to justify regal positions (in Glorantha this obviously becomes a bit more complicated what with Heroquesting and all). The Norwegian kings claiming to descend from Freyr, the god of agriculture (via an ancient (to them) bloodline in Sweden), is a famous example. Another, more recent example, come from the various Nilotic peoples in what is now South Sudan - where patrilineality is practiced heavily, but sometimes the gender of an ancestor is "discovered" or "re-remembered" to be a male rather than a female, which allows for what would otherwise be an exogamous group, to be treated as part of the clan or bloodline (otherwise the descendants of an ancestors sister would belong to her husband's bloodline, of course). I strongly suspect - or use as headcanon - that Gloranthan societies would also be open to similar creative moves for the sake of social cohesion and accommodation. I actually interviewed a number of people who'd defied traditional caste-regulations in rural India, and while their experiences varied, and all were tough experiences with disapproval for their families ranging from dismissive to outright disowning them, the ones who were hellbent on getting their partner in marriage did prevail, however "impossible" those unions were on paper (or in the collective mind of the caste-group, rather).
This leads me to the topic of marriages within clans. With the perspective of "bending the rules" in place, one could for example imagine a clever pater/mater familias, chief or other authority figure, asking a member of a different clan to act as a sponsor or patron of whoever wanted to get married within the clan, for example. One of the unlucky pair of star-crossed lovers could appeal with the sponsorship of a out-of-clan relative, maybe matrilineal even, to get things more palateable to the laws/norms. Maybe it would require a formal adoption rite to get an external affiliation, or a form of wardship, or just a token stay in the household of the sponsor or something, but ultimately if there is will for it, it could probably be pressed through. The force of the lovers' personalities, the standing of their families (ironically, higher-standing families might not make this easier, possibly the opposite), and external contacts and so on would all be factors playing into this.
I suppose the roleplayer's perspective here would be "storyhook", but since I'm a bit new to this world of gaming, I tend to think of it just as ordinary folks figuring out how to make life liveable when confonted with social strictures.
Since this is my first post here, I hope I did not commit any faux pas, or said something irrelevant or what have you. I'm enjoying the lore-side of Glorantha immensely, it's clearly a world that's been crafted with a great deal of anthropological insight.