If they really had some kind of romantic/sexual relationship with each others in the past, it's pretty sure that they didn't express it too openly and only kept it with themselves. I don't think that anyone would know about their feeling, not even Vodka nor her sister, since Akemi advised her to find herself a boyfriend... Imagine Shiho and Akemi's talk at the coffee like this....
Akemi: You should relax and find yourself a boyfriend
Shiho: I'm going out with Gin sis....
Akemi: ...