Who is Frances Owens from Practical Magic and what is their importance?

Frances Owens is one of Sally and Gillian's aunts who takes them in after their parents die. Frances doesn't believe in tipping and tells the pizza delivery boy not to expect a tip for his services. Frances is given a sapphire colored scarf by Ben when they first meet and, although she pretends not to be impressed, she puts it on while no one is watching.