There is another theory about Nomon, one of the three moons, being the Sibling. Link here.
It is actually possible for Vessels of Shards to have children, as we know from a WoB.
Additionally, we know that Honor and Cultivation were lovers...
Of course, a followup question to the first WoB asking if we've seen any offspring of two Vessels was RAFO-ed, but if it possible for them to reproduce, then it is likely that there are, even if we haven't seen or met them yet.