Well, in my exampleyour twin does not exist until you go back in time, at which point the entire universe splits in two and goes in different directions (albeit very similar). Your twin would of course NOT exist if you kill her grandfather before her father is born...
But the idea of a multitude of universes is not merely gobbledygook, it has received serious attention from many scientists. There is something called"many worlds" theory which argues that at every moment a decision has to be made on a quantum level (say, should this electron bounce this way or that), the universe actually splits and each copy takes the opposite direction, so that both things actually happen.
I didn't say it's credible, though.
