In Podkayne of Mars, Clarke leaves Poddy a cryptic note stating that he was going to rescue Girdie. Poddy is unable to reach Girdie and we find out later that she is not one of the kidnapped victims. Was this a trick to lure Clarke? Was Girdie in on it too?
In chapter twelve, Clark has been missing for three nights. Poddy finds Clark's note. Girdie has been following her normal routine, leaving work only a half hour before Poddy thinks to call her. That Girdie had 'gone shopping' instead of being available for the call seemed to be more a function of cutting Poddy off from support during the climax of the story rather than suggesting sinister motives on her part. Since everything is in Poddy's P.O.V. except the ending(s), we don't know how Clark was convinced Girdie needed rescuing.
From my reading, Girdie was totally uninvolved with the political intrigue aspect of the novel. The character's function was the world-wise older-sister role model. There was no hint she was one of the bad guys.