Maude told Arthur that no one's called her "Maudie" since she was a teenager by her Aunt Gertrude. On her first appearance on All in the Family (1971) she referred to herself as "Maudie".
Arthur tells Walter that he's an only child. In a later Season 5 episode, Arthur's twin brother Arnold makes an appearance. In that same episode, Walter reveals that he'd already known about Arnold, as Arthur had told him about the twin brother back in the Army.
In this episode, Arthur has a granddaughter, and he isn't estranged from his son and daughter-in-law. In season six, he has a grandson he has never met because he hadn't seen his son in ten years. His son had been a Vietnam War draft-dodger living in Canada since 1967. Arthur excommunicated his son when he refused to be drafted into the armed services.