Why was Adeline upset with Third Brother?

Adeline is upset with third brother because he allows the others to control him.... he follows their lead.

I lifted the glass and turned to Third Brother, my ally, knowing that he would never fail me. “Can I drink this?” I asked, confident he could be relied upon.
“Of course! Congratulations! We’re proud of you!”
Convinced, I took a generous sip of the ice-cold drink. The disgusting smell of urine hit me like a mighty blow.

Did Third Brother truly understand what he was up against? By wanting to have things both ways and straddling the
fence, was he aware that each compromise would chip away at his integrity? Yes! It was the loss of the nicest parts of Third Brother that saddened me.