So some BTS photos from 2x15 are also out and
this one also shows the Downton Abbey actress (I think) who plays a nurse or something close to Snow's family in FTL up in the clock tower with Cora and Regina as they're confronting Snow and Charming (who have the dagger). Looks like Regina is holding a possible heart in one picture, so maybe Cora took the nurse's heart and made her poison or otherwise cause the death of Snow's mother? (Setting up Regina becoming Queen and Snow's step-mother down the line.) Snow certainly looks horrified in several of the other promo pictures.