Congratulations!!
My sister and I are a little over 3 years apart. Might just be us, since we are both quite stubborn (we come by it honestly!), but we didn't really get along until she moved out and I was over 18.

She beat me up as a kid; I worshiped the ground she walked on as we got a little older and that annoyed her to no end. Once we got older and had our own identities & personalities we both realized that the other one was actually a pretty cool person.

So I think whether or not they will get along has more to do with their individual personalities than age difference. It was good to have a sister growing up, though. Even when we didn't get along with each other we
always had the others back when it really counted.
