Firstly, when we do get together we keep getting asked if we are sisters, or specifically, twins. I even overheard niece on the phone to a friend, telling friend how much her aunty and her mum look alike. The lady who did our manicure asked if we were twins. A complete stranger in the manicure salon came up and asked us if we were sisters, and a couple of times in other shops we were asked the same question.
But here's where it gets really freaky. See, I spent ages searching for a particular gift for my sister's birthday (a digital photo keyring) only to find she had bought me the exact same thing. And then we discovered that we had both bought Sarah the exact same birthday card. (cue Twilight Zone music).
We both manage to get lost easily too, although I think that's more of a female thing than a sister thing specifically.