Without any examples of how your brother behaves or how he's treated badly, it's hard to answer for sure, but I've seen similar things with different causes.

1. Some people are naturally attracted, for unconscious reasons, to people who will treat them badly.

For example, I know a wonderful young woman who was abandoned by her bio father and verbally, emotionally and physically abused by her step-father.

Since her very first boyfriend, she has picked men who treat her badly, verbally abuse her, exploit her in every way and leave her lonely except when they want sex.

She sees the pattern, relates it directly to the way she was treated by her father and step-father, but she hasn't been able to stop it.

She was also verbally abused by a series of step-mothers. She has very testy relationships with her women friends.

She treats people well, but she seems to be attracted only to people who replicate the abusive people in her past. She says she knows she irrationally believes if she can make a no-good boyfriend or a mean-tempered woman friend love her and treat her well that it will be just as if she'd conquered her desire to make her father and step-father and step-mothers love her. Insight has done nothing to change her behavior or the men and friends she chooses. My heart aches for her.

Was your brother abused as a child, either verbally, emotionally, physically or sexually.

2. Some people provoke others by testing them, making sarcastic comments, arguing passive-aggressively, purposely pushing buttons to get a reaction, even doing it in a seemingly nice way, with jokes and funny comments.

I've known people who did things -- probably with unconscious motivations -- to remain the victim, the person who's treated unfairly, who's put down and hurt. They seem to gain a significant feeling of personal power if they can trigger the people around them into losing their cool, shouting, or saying unpleasant thing or stomping out of the house. I don't know what causes this, but it's common in dysfunctional families for one or more people to get their self-esteem and personal power by enabling or triggering other people to act badly. The person feels better about themselves in comparison to the person who lost their cool. This behavior is common in people who grew up with alcoholic or addict parents and who developed traits of co-dependency as survival skills.

Did you and your brother grow up in a messed up family with people who may have abused alcohol and drugs, food, gambling, porn or other dysfuntional behaviors.

I don't know if that information helps you. You're to be commended for seeking therapy to understand and improve your own behavior. Perhaps your brother will follow your example.