The more I think about this, the more I think that there is definitely something to this. The mosaic is in the Holy Urithuru engine room, the heart of the beast more or less, and there were always no more than 3 bondsmiths, so the depiction of 3 avatars of divinity seems very important.
I also think that the referral to the Sibling in plural is important, like @Billahollic noted.
I also think that the reason given by the oathgate spren to not let Shallan and co transition back to the physical realm because "they are locked by word of the parent" as noted by @Ryder is important too.
So preface aside, here is the theory. We know from Brother Kasbal's very slick demonstration, that some of the major cities on Roshar are built along cymatic patterns. The reason they are built around Cymatic patterns is because the actual physical geography of the land has been created in a cymatic pattern. We know that there are cymatic patterns for Kholinar, Vedenar, Thaylen City, and Akinah (in Aimia). We also know that there are Oathgates in those cities as well....So here is the theory, what if the Oathgates and Urithuru itself are remnants of Adonalsium's original investiture in Roshar, and that some cognitive shadow of Adonalsium is the focus for these separate pieces of Adonalsium's investiture just as the Stormfather is focus of the splinters of honor that are honorspren? Might the sibling then be the primary focus for these remnant splinters of Adonalsium (like the oathgates, possibly the greatshells)? If this was true then the Sibling bondsmith would not only be master of the Living city of Urithuru, but could also be master of the Oathgates as well (and possibly be able to utilize the investiture focusing potential of the Rosharian greatshells too boot).
Another possible reason that the the Kholinar wind blades and the Tower city of Urithuru are similarly described as being crem covered might be that prior to the shattering of Adonalsium the High storm reached Urithuru as well. It's a pre-shattering feature of Rosharan ecology that the rain contains crem, a vital vector for the nutrients necessary to sustain life in a world without soil. The salient feature of Vedinar that is mentioned (which is probably the cymatic manifestation of Adonalsium's original creation) are the shelf like ridges, might these also be covered with crem (which would imply, in this context, a certain similar AGE, a certain definitive time of creation that was in the far, far distant past?)
So, my speculative money is on the Sibling being a focus for the other leftover splinters of Adonalsium on Roshar, and further that we've seen the splinter onscreen in OB (Chapter 19, Subtle Art of Diplomacy):
I think that the references to the Sibling sleeping are merely that it has reverted (like Syl and pattern) to a mindless state due to it's lack of a bond. From this scene and the Kaladin/Shallan chasm scene where they see something similar, it seems that somehow the Stormfather is protecting it within the nexus of the highstorm. We know from Kaladin SF scenes that at the center of the highstorm you really are in the spiritual realm (because of the pressure of investiture most likely), so this is probably how the very protective and Brotherly storm father has been protecting the sibling since the last bond smith that was bonded to him/her/them died.