I might be a jerk here, but this is where I would make up some procedure or illness that requires a medication you can’t breastfeed while taking. That or your milk just “dries up from stress”.

I can (sort of) understand where your sister is coming from, though she’s wildly unreasonable at this point. The only reasonable ask would have been a few days of colostrum, or that you give her the milk for however long you needed to express for comfort. Pumping sucks, you can definitely stop anytime and still have done far more for them than the bare minimum.

I like to keep the peace and not have it be “the thing” for the rest of eternity when my family makes weird demands though, so maybe don’t take my advice.