I really like the idea of the Sibling being related to stone for several reasons.
1. This provides three powerful spren for three fundamental forces/objects in Roshar. The storm, living beings and the ground.
2. Urithiru is carved almost like it was out of one piece of stone (I can't find the exact quote but the stone, strata and general make-up of Urithiru is mentioned in OB many times)
3. The Shin worship the Stone as holy, but there is no reason for this in our current understanding of Roshar.
Someone likens Urithiru's passageways to the tunnels through the Windblades in Kholinar - this may be where the Sibling has gone to slumber?