Is this putting any undue stress or problems on your mother?
Not to excuse your sister’s behavior, but my experience with narcissists is that you can’t teach them to be better or “fix” them unless they want that, and that’s rare. Which usually means an you can do is make the best out of what you get.
Sometimes this means cutting them off completely, but when there’s kids involved like that, if that’s the way to get to see them, that’s all you can do. Just make sure you (or your mother) have your own clear boundaries and stick to them when things get out of hand, as they often progress to eventually.
People mention eating it whole… I at least shave the hair off it first.