Hansel and Gretel's mother is the Woodcutter's wife and the mother of both twins, Hansel and Gretel. She died a few years ago before the curse of the Evil Queen. At Storybrooke, she is known under the name of Dory Zimmer.
Before the Curse
A few years ago before the curse of the Queen Regina, the mother of Hansel and Gretel died in unknown circumstances. So, she left behind her her two children and her husband, the Woodcutter. ("True North")
After the Curse
Ava and Nicholas are seen having breakfast in Emma's apartment, and the latter is seen having a talk with her roommate and friend, Mary Margaret, asking her if she knows the kids, and whether they go to her school. Mary Margaret claims to have seen them, but adds that she no idea, no one did. Emma opens up a file on them, and tells her friend that the teens' mother was a woman named Dory Zimmer, who apparently died a few years before. The name doesn't ring a bell with Mary Margaret, and Emma tells her that no one seems to have known or remember Dory. ("True North")