Is it normal that I'm not bothered when friends ask this?

Perhaps he was afraid to judge his sister in that way. I don't know many people who openly admit if they consider those of their own blood to be "hot". Try using the adjectives "pretty" or "attractive" instead, or questions whose answers might hint at a hotness level, such as, "Does she have a boyfriend?" or "Is she a dirty whore?". Actually, your friend might be just as offended by that last one.