Well, I tried Googling for the origins of the expression, and couldn't come up with anything, apart from the fact that it's an English expression.... surprising. I must admit I'd never thought about it - it's just one of those phrases you've been using all your life - you know what it means, but have no idea why!! Despite being true born and bred English, and never budging therefrom all my life I have NEVER heard anybody use a phrase about "Mrs Binn's twins". I can only assume that it's one of those regional things - whereabouts do you hail from Trubrit???