It's a nice theory but sometimes simpler, less glamorous solutions might hold the truth. There are more than 6 "sisters" visible with the naked eye, but most people normally can't see them. It's more believable to me that the "missing" 7th sister is just the brightest of the less visible ones. Those less visible would certainly have been used as a test of prowess for determining the keenest-eyed, especially among ear…
Let's consider your rule itself, though first modifying it to the real occams razor which says the simplest answer is probably the real one. Simple is probably more measurable than glamor anyway! So what's the simpler of the two explanations? A complex variation of the story as we now know independently arose in multiple cultures and then everyone forgot the same detail (find the seventh star it's still there), or we've been carrying this story with us since the beginning of humanity? Honestly the later sounds simpler to me. It's breathtaking but sounds entirely plausible so why not?