The closest I can think of this scenario is Buck Adams and his sister Amber. They kept their brother/sister relationship private and no one knew until a company tried to pair them together for a scene. Another weird story I've heard is that Aspen Stevens is supposedly the niece of Jim South and he started her in the biz (I have no idea if its true, but have heard it more than once, so using the where there is smoke, there is fire theory...)
But, of course, there are some insiders here who probably have better stories.

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