"Sister Act" is an exclusive comic story in TV Guide magazine's Comic-Con edition. It was released digitally on July 6, 2015 and on July 7, 2015 nationally.
Supergirl arrives to a Kerfuffle terrorizing National City and its citizens. Confronting the Fort Rozz escapee, the kerfuffle attacks her, firing it's horn at her, which explodes on impact. As she didn't think he could grow another horn quickly, Supergirl charges at the kerfuffle, pushing him into the sewer to be apprehended by the D.E.O. Afterwards Kara is berated by her older sister Alex Danvers, over her earpiece, for not heeding the latter's advice of watching out for the kerfuffle's horn. Though Kara thought she was merely kidding about the horn, Alex tells her that she doesn't kid during mission briefings, and that Kara needed to prepare better before going into battle, reminding Kara that she had been fighting the Fort Rozz escapees for over two years, whereas Kara had just started.
Telling her sister to hush as she could hear something, Kara flew her way up onto a balcony of an apartment, finding a girl named Tracy crying. Reassuring her that the kerfuffle couldn't hurt her, Kara asked whether or not she was alone. Tracy explained that her sister was watching her, but she had to go to the store, and though she had told her sister that she'd be fine by herself, she got scared when the kerfuffle began attacking. Kara offered to sit with her until her sister returned, though Tracy nervously asked what if she didn't return. Deciding to give her some advice, Kara told Tracy that she had a sister too, and although her sister could be really annoying, always telling her what to do, and telling her that she should listen more, she only did so because she loved her, and wanted to keep her safe. As that's what big sister did, and that Tracy's sister would be coming back. Arriving at their apartment seconds later, Tracy's sister apologized for being gone so long, though Tracy assured her she was okay as the two sisters embraced each other, telling her Supergirl had stayed with her until she had came home.