How do you say hi sister in Japanese?

Ko-(knee)-chi-(wah) She-my

(then add [-san(formal or polite)] or [-chan(common and familiar)]

So, it would be

konichiwa Shimai-san!

or

Konichiwa Shimai-chan!

And only when you are a very young (still in early-middle grade school) or a Girl (of any age) you can say

Konichiwa Shimai-saaaaan!

or

Konichiwa shimai-chaaaaan!