My current main theory is that Zon-Kuthon dies and Shelyn becomes more distant overall in grief or in pursuit of answers.Zon-Kuthon is a good bet, since there's a real meta problem of "how do we solve the Nidal problem." Since some of the stuff that's been written about Nidal is dark to a degree that Pathfinder doesn't really do anymore. It just seems weird to say "well, Cheliax doesn't do slavery anymore" (a good change,, IMO) when their neighbor to the north is still kidnapping people to slowly and gruesomely torture to death in service of their warped god.
If Zon-Kuthon gets bisected by his sister, that sets up for Nidal to still be an occasionally horrifying place (There's a lot of vampires around) but less of an overwhelmingly horrific place.
There's also the possibility that in her grief, Shelyn doesn't just become distant, but also starts committing unholy acts in the name of finding answers, bringing him back, self-flagellation/guilt- whatever fits the narrative of how he would have died.