AITA for telling my cousin that his dad talked badly about him behind his back?

I (16M) have a cousin, whom I’ll call Kanye, (16M) who is an older brother to a golden child (15M). My family got together recently for a family dinner, that is everyone except Kanye.

My uncle, Kanye’s dad, was asked about his boys. He went on a big rant about how much of a golden child, how skilled, talented, and amazing Kanye’s younger brother was. When my uncle was asked about Kanye, this is what he said:

“Oh Kanye? He is a trainwreck. In fact, he is the exact opposite. He has a 3.5 GPA (unlike my golden boy), sleeps in until noon, is not talented. For my golden boy, the sky’s the limit. But, for Kanye… I just hope he stays out of jail.”

I felt like this was blatant favoritism, but didn’t say anything about it. Later, Kanye asked me about the dinner. We discussed how it went and the topic of his dad talking badly of him came out. I fully admitted everything he said and asked Kanye to keep this between us. A day later… I get a text for my Uncle, Kanye’s dad totally enraged asking: “WHAT DID YOU SAY TO MY BOY?” Later, I got text from my dad and other family members telling me that I was incredibly wrong for telling someone a private conversation behind their backs. Am I the asshole here?