I feel like one of the reasons kismesitude is the way it is might be for like… historical reasons? Specifically with regards to ancestor folklore.

You look at the way Vriska specifically mirrors her ancestor and a sort of pattern emerges; she strikes it up with Terezi because there was a known rivalry between Mindfang and Redglare. She strikes it up with Eridan because Mindfang and Dualscar had a known rivalry.

A similar trend exists in real-life folklore; tribes and clans (which on Alternia, I suppose, consist of a single person) are bound together by shared ancestry - i.e. the ancestors of their families were lovers - or put at odds by rivalry - i.e. the two families might have an ongoing feud between them because of something that supposedly went down between their family heroes (which is something evoked in Homestuck itself, where trolls tend to gravitate toward killing those who killed their ancestors).

When you’ve got really definite ways to refer to those relationships - matespritship for lovers, kismesitude for rivals - it becomes really easy to define the way your bloodline is tied with the bloodline of another person. Me and Troll X are destined to cross paths because our ancestors were lovers; me and Troll Y are intertwined because our ancestors had a famous fight.

It fits in with the sort of ‘genetic destiny’ highbloods seem to believe in pretty well; wherein Eridan acknowledges that the concept of an ancestor does have a genetic basis, but that there’s a big destiny aspect to it as well. I think because of this, a lot of ancestor stories that were literally just about normal rivalries get labelled and identified as ‘kismesitude stories’ for ease of understanding; Redglare and Mindfang were clearly never in an actual intimate relationship, but if Vriska claims they were in a kismesitude, she can found a genetic basis for her connection to Terezi.