Or if he’s even there if he’s George’s kid brother he dragged along.

That said I think it’s clear it’s a vision sequence specifically created by Clarence based on the limited story he saw from George’s life. Mary jokes she married George to keep from being an old maid earlier. Clarence took it literally and made her that in the sequence. Also his distorted view of Potter has him literally taking over Bedford Falls with Strip Clubs and calling it Pottersville. Clarence is portrayed as a very simple and naive angel which supports this theory, In a way it’s kind of genius of Capra.