In that sentence, 長男(CHONAN) means the eldest brother, 一男(KAZUO) means a male first name, and 兄ちゃん(NIICHAN) is just added as a Japanese habit to call his big brother's name. You may call your parent mom or dad instead of her/his first name. In Japan, we always call our elder brothers/sisters (お)兄ちゃん/(お)姉ちゃん instead of their first names, or their first names along with (お)兄ちゃん/(お)姉ちゃん, just like you may sometime call your aunt Aunty or aunt Jessy. I should add we have no way to call our younger bro/sis in these way. (We don't say 一男弟ちゃん.)

Thank you, Orsinia, but how would I know just by looking at it that 一男　is pronounced as かずお and not いちなん？