who's myrtles sister

Myrtle's sister's name is Catherine.

From the text:

"The sister, Catherine, was a slender, worldly girl of about thirty, with a solid, sticky bob of red hair, and a complexion powdered milky white. Her eye-brows had been plucked and then drawn on again at a more rakish angle, but the efforts of nature toward the restoration of the old alignment gave a blurred air to her face. When she moved about there was an incessant clicking as innumerable pottery bracelets jingled up and down upon her arms. She came in with such a proprietary haste, and looked around so possessively at the furniture that I wondered if she lived here."