It's Not Cheating If She's Your Sister. (фемслэш)

Kara and Alex had a close relationship. Closer than most foster siblings, Jeremiah once said. Closer than even his and Eliza's marriage and the two were madly in love, her mother voiced.
It was like they shared a secret only they should know, a secret that was theirs alone and no-one else should be let in on. A vital secret, much like their connection, they couldn’t breath in if the other was not close to exhale.
It shouldn’t really have come as a surprise, it was hidden in plain sight, played aloud for all to hear. But much like the obviousity of love itself, it’s not until Maggie Sawyer comes into the pictures that their secret is let known.