Captain Marvel #18 has a first appearance after Carol becomes the new Accuser coming out of the Empyre story line.
So if you don't like spoilers, turn away now.
These spoiler panes take place right after Carol is sent on her first mission as the new Accuser.
So there you have it, first full appearance of Carol's half sister. I can also confirm that she's in the next issue after Carol dumps her off at her apartment on Earth where she goes on to save a girl when she ventures out into NYC.
So maybe Marvel has some future plans for this new character as I don't see her going away anytime soon. What becomes of her is still uncertain.