Sometimes you get more mileage with being softly critical than judgmental. Being judgmental will put your brother on the defensive. By judgmental, I mean “I hate her, I can’t stand her, she’s unbearable, she’s whiney.” Instead focus in on discussing traits that you’re surprised are compatible with your brother. Spend some time figuring out why a person would enjoy spending time with the person.
Keep in mind is that you know your sibling as a sibling and family member. People often have a different self-image of themselves that can be very different from the way they were raised. There is some separation from family as you become an adult.
My younger daughter was very attached to her sister’s old boyfriend. She cannot warm up to her current boyfriend, who is very good to my older daughter, and is a great boyfriend. He’s just not friendly towards my younger daughter, and is somewhat judgmental. My older daughter told me some things he said about her sister; they’re true, but are not the whole person. Although he keeps his thoughts to himself, he avoids her, and is not inclusive towards her. On the other hand, the old boyfriend was not the best boyfriend in how he acted towards my older daughter.
What you see of the person is not the whole person.