Well she despised magic users, but she didn't hate them: not like her father did. She didn't mistreat Leetah and quite liked Rayek befor he tried to kill her and her tribe.
So...she may have left in a huff or for a specific project and not been able to come back.
1) Go-backs used to pool the kid raising duties, so leaving Teir to his Dad would not be particularly unusual or uncaring.
2) elves grow into teenagehood very quickly (the look of Cutter at 16 (when he becomes chief) and the narrator's comment about this during the twins' childhood in the Sun Village
3) when Lerringen (sp???) speaks of killing that elf with the braid, he is quite old
4) we don't know Teir's age and he may be quite a lot younger than Ember. Remember the huge age difference between Leetah and Cutter. (hen they met he was in his early 20s, she was several 100s years old) and the recognition between Scouter and Tyleet.
Maybe we'll get to know in the final quest, but I think she didn't ever come back mostly because she was dead.