We're Sisters, But?

This story starts when two persons in opposite meet each other. Charlotte Austin is a cool-headed person who quietly thoughtful, a veterinarian from a coastal town, she built her life around logic, put aside feelings to simply keep her life in peace. Flip of the coin, there's a dedicated elementary school teacher, gentle but fiercely passionate and very compassionate named Engfa Waraha.

Then, their paths were crossed. From their first conversation, something vibrates between them. Unknown familiarity, warmth, strange sense of coming home to someone they've just met. They bond quickly, too quickly.
However, they promise themselves they were nothing more than sisters. Deep down, they both knew sisters don't look at each other the way they did, don't cross the lines they crossed.

So, they're sister, but?