Great-Aunt Beatrice is the Petunia Dursley to Amy and Dan's Harry Potter, the Count Olaf to Amy and Dan's Baudelaire orphans, the wicked stepmother to Amy and Dan's Cinderella (and Cinderfella), the... okay, you get the point. She's bad. She's the bad adult that's all the rage in children's books these days. She's a bad guardian, wears bad makeup, and is a bad driver. She has no redeeming qualities whatsoever. Bad bad bad.
To make matters worse, she's also keeping secrets. She might just know exactly what Grace's ultimate power is. This is alluded to when she says, "You'll take the money and wash your hands of my sister's ridiculous scheme for finding the—" (3.51) but she stops before revealing exactly what Grace has hidden.
Perhaps we shouldn't blame her. Her sister's name was Grace. Her niece's name was Hope. She got stuck with Beatrice. At least she wasn't named Pestilence. Sure, there's no excuse for firing so many au pairs and threatening to either take Amy and Dan's inheritance or hand them over to social services, but we don't want to judge too harshly before we know her side of the story.