I am so angry on your behalf. Good grief, what is wrong with her! Please know it is her, not you. I have SILs who live with my MIL and neither of them would EVER speak to me like that for any reason, but especially not if I was in the middle of actively helping their mother.
I would be doing nothing.That might be the wrong response and attitude, but I would back all the way up and do nothing to help.
She is a jerk.