Who is Mildred from Hocus Pocus and what is their importance?

Mildred is Eugene's mother-in-law. For all practical purposes, she is a doppelganger, or mirror-image, of Margaret. Both are nearly catatonic when they are around Eugene. Both are reclusive. In fact, Mildred largely acts as a visual echo of Margaret, suggesting the line of hereditary insanity is long and tragic in their family's tree.