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I don't remember the specific incidents, but the general set-up sounds a lot like Nicholas Fisk's Grinny, in which a mysterious old woman shows up and hypnotises the child protagonists' parents into believing that she's their Great Aunt Emma. It was a novel originally, but there was an animated adaptation as part of CBS Storybreak.
Edited by PaulA

This is an animated movie that I saw in the early/mid 90's on one Saturday afternoon, probably some kind of ABC special. I didn't watch the whole thing, but from what I remember:
The protagonist was a young girl and the antagonist was an old lady that was likely some kind of cyborg or gynoid — she had Hypnotic Eyes that she used to pass herself off to people as their Aunt (name forgotten), after which they would immediately act as though they knew her. She didn't hypnotize the girl, but she did hypnotize the girl's parents.
Later, the girl and the "Aunt" are at a grocery store. The girl talks about not knowing what the prizes are in cereal boxes, so the Aunt starts opening the boxes in the aisle. When a worker comes by to intervene, she hypnotizes him into doing the same.
The last scene I remember involves the girl spying on the Aunt from above (in a treehouse?) The Aunt is investigating a cactus, gets some needles stuck in her hand/arm, and then deposits them in a hidden compartment also in her arm.
Edited by UltramarineAlizarin