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

Yes, it is, and she feels safe that you would answer her honestly to avoid buying it wearing it out and making a huge fashion mistake. It is all about the trust she has in you.

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

Why not? Many women seek various opinions on how they dress. The are looking for honesty and / or reassurance on what they are wearing.

Yes, very normal

I deon';t recall either of my sisters ever asking me that, but on the face of it, it's innocent.

I wouldn't ask mine, but whatever