Milissa Wilcox is the direct ancestor of Arlene Wilcox and her twin sister, according to The Scooby-Doo Show episode "To Switch a Witch".
Milissa lived from 1733 to 1788. She was tragically killed by the town of Salem because they believed her to be a witch. She swore she would rise from the dead and seek revenge.
The Scooby-Doo Show
Season three: Hundreds of years later, Arlene's twin had set her up by making it appear that Milissa had returned as the Ghost Witch of Old Salem through Arlene, since she apparently looked like Milissa. Arlene's sister knew that this would send the townsfolk on a witch hunt, driving Arlene out of town and leaving her the family inheritance.