No according to the book (unless I'm mistaken) Sally is older by maybe a few months or a year even?

I think Gillian was the elder sister.

I'm going to assume that they're like my sister and I. We were mistaken for twins while growing up despite me having blond hair and her having black hair until we turned 7 and our hair turned brown and then we didn't look a lot a like, but you could still tell we were sisters. I'm exactly 1 year, 1 month, 1 week, 1 day, 1 hour, and 2 minutes older than my younger-middle sister. (I have another younger sister) I have to say it that way because my mom was so proud of her "accomplishment" and then the doctor messed up and made it 2 minutes and she just loves to tell people about that, so my sister and I repeat that whenever someone asks who is older and by how much. =P

In the book, they are 13 months apart.

It's explained in the extra features on the DVD about the hair. According to the producers it's typical in witch families for sisters to look nothing alike, hence the obvious hair difference.

Okay, thank you.

It's not mentioned in the movie but I don't think they were twins. If I had to guess I would guess that Sally was the elder sister by a year or two.