Since my brother lives separately, if we talk about the acts that he's committed behind him or discuss about his qualities and bad habits, will that be counted in gheebah?
He's my elder brother but rest of the family members are older than him and they discuss about his life and his behaviour in my presence so, am I allowed to comment or I should keep quiet?

If what they are saying they would not say it to his face or in his presence you are not allowed to partake in such gathering. Gossip is haram abd it should not be entertained