The girl sitting on Captain Marvel is his sister, Mary Marvel. This is from the god awful series Final Crisis, where writer Grant Morrison basicallly takes a giant shit on everything in the DC universe.

Mary Marvel was the sweetest, most innocent character in all of comicdom. She could make the kids from Archie look like a bunch of crack whores by comparison. So Morrison decided to have her turn evil by joining the evil New God, Darkseid.

The Marvel family get their super powers by yelling the name of the mystical wizard, “SHAZAM!” The “dirty magic word” Mary is referring to is that she now says “DARKSEID!” to get her new evil powers and costume.

The lines of dialog don’t really make sense in any context, as Morrison likes to throw out random ideas without explaining ANY of them. When other writers try to fill in the gaps left by Morrison (the series Countdown to Final Crisis was supposed to do that), Morrison will bitch and complain that the writers did it wrong, but fails to elaborate on how to do it the right way.