Sibling rivalry: doom and gloom or fun and frolics

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The sport world is littered with successful siblings: Venus and Serena Williams, Olympic rowers Jonny and Greg Searle, and Michael and Ralf Schumacher, to name a handful.

Our own horse-powered sports have numerous examples of their own. So does making it a family affair foster success, as siblings push each other harder and pool resources and experiences, or can competition test even the closest family ties, leading to tack-room tantrums and showground showdowns?