Gaslighting is abuse. A legitimate case of not remembering something is not the same thing. (And she really seems like a ditz so her forgetting she said it seems far more likely here.)
as someone with serious memory problems im seconding this. while i dont think i'd forget something like that, i have a habit of forgetting things other people feel are important. sometimes your brain just dont work right for remembering some shit
The situation here seems less like gaslighting (which tends to be sustained and deliberate) and more like denial based on it being an offhand comment that she's no doubt forgotten, which was totally at odds with how she's feeling now. Indeed, she's come around so much she's having trouble imagining that she wouldn't accept her sister.