The venthyr travel long distances through enchanted mirrors. The keys we look for are not as we have in Bastion, but rather shards of a mirror that will lead to the sinstone's location.
Based on the intel you found, we know which three houses hold these shards.
Primrose, Darkvein, and Sinfang.
Go to their manors and retrieve the shards. We will find the frame to piece them into.