I have a friend who is male and very early 20s. He revealed that his brother, who is mid 20s and 5 years older, has humiliated him throughout his life, including at a job they both worked at, to the extent my friend left the job. A recent example is that my friend, who has severe dyslexia and ADHD (and is incredibly bright), is terrible with names and was trying to recall a name, and his brother began to humiliate him for being stupid, and their father stepped in to stop him. Apparently this is constant and has gone on throughout their lives. Yet when they are around each other my friend somewhat defers to him and I can see looks up to him a bit. (The brother went away to study and has a master's degree which is unusual for the family and the place I live, not UK). I have not seen much of the brother but he seems friendly, if with a very slight arrogance, but I may be projecting because of what I know.
It's a small town so there aren't that many people to make friends with, but I wondered whether this would make you think twice about becoming friends with someone when you had that private information. The brother does it freely in front of the parents but conceals his behaviour from his own friends, so the brother hasn't humiliated my friend when his own friends are around.
I suppose i am sensitive to it as I have a similar relationship with my sibling. They are not very nice to me, but I sort of have a hero worship complex about them. My sibling is very popular and successful and has a wide circle of friends.
So, would you avoid any kind of friendship with the brother, or treat them like anybody else. If avoiding a friendship, should you be pleasant and civil to them when you see them, or a little unsmiling and cool.