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

Gillian is the younger sister of Sally. They are very much opposites. While Sally is structured and conservative, Gillian is very free spirited and believes that it is a burden to live in one place for too long. Gillian leaves home at a very young age and moves around the country. She is married several times but never stays with one man for very long. When Gillian comes to visit Sally, it is because she is frightened thinking she has killed her abusive boyfriend, Jimmy. It is later discovered that Gillian did not kill him, which has been weighing on Gillian for a long period of time.