The World Beyond Ours

Luna Delphinus knew something was wrong the day she saw her sister get abducted by aliens. Due to her family's dismissal of her even having a sister, Luna is forced to pretend the incident had never happened. It's not until ten years later, and Luna is in high school, that strange things begin happening again.
She finally starts to find some answers the day she meets Luxite Cygnus, an outlaw in every universe except Earth's. Luxite is banned from Earth's universe for breaking the biggest truce in the galaxy, and Luna is pulled along with him in hopes of finding the truth about her sister's disappearance, stopping a war amongst the galaxy, and the reason why only she can see Luxite and his kind.