Loanword from English, used to refer to an older brother, often in a casual or affectionate way. Less common than native terms like 兄 (あに) or お兄さん.
He is like a big brother to me.
Big Brother; dictator; despot
From George Orwell's novel 'Nineteen Eighty-Four'. Refers to an all-seeing, authoritarian leader or a surveillance state. Often used metaphorically for oppressive government monitoring.
In that country, Big Brother is watching the citizens.