Is it normal for a sister to ask ner brother's opinion on a dress?

I think she trusts you to give her the truth, not what she wants to hear (like a "friend" might do), not flattery (like a "person trying to get into her pants" might do), but just tell her the truth.

You've seen her over YEARS, so you also know what she's looked better in, etc.

Yes. My sisters used to ask me, when we were in our teens and lived at home

I wouldn't ask mine, but whatever

Yes, very normal