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His Canadian father just about to go back to Canada, and perhaps money was so tight in those days, his mother saw no other option.
I have a good friend, in his late 60's, who was told on his 13th birthday, that the person he thought was his Mum was really his Grandmother, and that his older sister was really his Mum. His Mother/Sister had a relationship with a German POW, in Bath after the war, and the shame of that was so great, that the family though that the only way out, was this extraordinary concoction of the truth.
I have heard similar stories from other sources.