Never Tell

Emily died in a biking accident when she took impact to save her twin sister. She sold her soul to forever watch over her family as a demon.
Emma, wanting to be with her twin again, attempted suicide but when she failed, she gained the power to see her sister and other ghosts like Emily.
But not all had done what Emily did for the same reason, as Emma would soon find out. And not all are like Emily, either, some are very tangible and some like twins, and a specific one demon is interested in Emily's sister.