Learn English – one word for “to take one’s side”

OP's brother is exhibiting extrafamilial orientation - normal behaviour for young adults of both sexes as they approach the age at which they will leave the parental "nest".

Perhaps, catharsis — M-W

2b : a purification or purgation that brings about spiritual renewal or release from tension

"She has learned to have her catharsis, take a deep breath and move on. … she does not dwell on the negative anymore." —Selena Roberts, New York Times, 24 June 2001

Wikipedia explains in detail:

Stages of social catharsis:

1. Directly after emotional effects, the emotions are shared. Through sharing, there is a reciprocal stimulation of emotions and emotional communion.

2. This leads to social effects like social integration and strengthening of beliefs.

3. Finally, individuals experience a renewed trust in life, strength, and self-confidence