They do show that Cera's mother was on the same side of the great divide as her father was. This however does not explain the continuing absense of Cera's mother in the sequels. We never saw a threehorn again that was certainly Cera's mum. We do not know for sure what happened to her, and an explanation of this is the least I expect from the movie makers if they come up with a new mother for Cera.
