I've never had a biological sister--only one or two virtual ones--so this "Sis" business is new and fun!
One of my "virtual sister" stories:
I have a friend who lives in Canada who I used to travel with often. EVERYONE would look at us, pause for a second, then say, "Ah! You're sisters!" EVERYONE! When we'd try to explain that we're just friends, they wouldn't believe us! We HAD to be relations. Do we look alike? No, but we had somewhat similar haircuts and both wear glasses. It was hysterical.