Am I overreacting?

I was talking to my sister tonight. She mentioned some acquaintances of ours and said "He's super skinny, but she's a whale."

I said "Gosh, wasn't that just the NICEST thing you've ever said." (Sarcastically, of course.)

My sister then replies, "What? I mean, YOU aren't THAT bad. They are huge."

I got pissed!!!! What the hell? How and why did my weight get brought up? She then tried justifying her response. I told her how upset I was that she said what she said. She never even tried to apologize. Am I overreacting??