My older sister is always comparing our children.
Oh wow, that's kind of rude.
Yes it is upsetting, because she alludes to her children being smarter and more well-rounded than mine.
Have you brought it up to her? Maybe she doesn't realize she's doing it.
I have not had a conversation with her about it. However, I know she does it on purpose just to get on my nerves. She has a history of getting enjoyment out of messing with me and now my kids.