You don't want to upset HER? This isn't just nastiness, if this were anyone but your sister we'd all be calling a spade a spade and say it's bullying.

Tell her to shut up and give her mouth a chance. If my sister were still treating me like this (not about weight but about other things) I'd not see her at all, no regrets. Same with my mother, who backed her up. You don't need people treating you like this.

(BTW, I'm a size 8-10, 5'11 and about 65kgish. Trust me, you're not fat. Cos if you are then I am and I'll have to cut back on food and no-one will be happy then.)