Before you look at
Webster_1913's definition, scratch your head, and say
Huh?, it is possible that the seventh sister had been seen before the invention of telescopes due to:
- Persons with exceptional eyesight. There are cases of people being able to see stars other people can't; the best known example is that of the binary Mizar in Ursa Major.
- The seventh sister was brighter once, and has since faded. This is the most likely answer. Both Native American and European mythologies tell the same story about one sister conceling herself out of shame.
- Space Aliens told them. Then again, maybe not.
By the way, the mythical Pleiades were named: