Now I know who was responsible for the modified ice mummies in 2e. (Was ice javelin a lucky guess, or was that detail added after the episode?)
Anyway, I think this is highly likely. Due to their limited numbers and inability to breed with other humans, I imagine that most Hyperborean couples are at least fairly close cousins. I don't think you can ignore Hyperborean decadence and/or desire to be transgressive either; some may have relations with their siblings even if there are alternatives.

Blackadder23 wrote:

(Was ice javelin a lucky guess, or was that detail added after the episode?)

That episode aired in the midst of Second Edition editing and layout. Whatever spell I initially had selected in that slot, I reached out to Jeff and said we absolutely needed to change it to ice javelin.

DMPrata wrote:

A delicate topic, I understand, but one that I think fits the sword & sorcery oeuvre well. Given that they’re a dying race incompatible with other humans, would it be much of a stretch to posit that incestuous relations betwixt Hyperborean siblings are not uncommon? (I’m looking at you, Cersei and Jaime.)

It certainly sounds reasonable to me, especially given the context of Hyperborean families staying holed up in their mountain fastnesses during the Green Death and its centuries-long aftermath.