Update: Oops. Turns out I let my enthusiasm mix with my ignorance and came to the wrong conclusion about Janis. Someone who knows way more about both of them than I (hint: we're married) informs me that, while Janis was influenced by Etta, both she and Etta were primarily influenced by Big Momma Thorton. I stand corrected.
h/t: The G Spot. Go there for more Etta vids.