Amy grew up in a broken home, with a mother she does not get along with most of the time, and had to raise herself to an extent. Karma provides the nurturing role that Amy never had.
Karma grew up constantly competing with her brother, feeling that if she did not live up to her parents’ expectations, they would not love her as much as they loved Zen. Amy provides the unconditional love that Karma never had.
They’ve known each other their whole lives, they’re always there for each other when it really matters, and they love each other more than anyone else on Earth. Don’t tell me it’s unrealistic for Karma to develop feelings for Amy.