Some TMNT stuff really isn't for little kids.
Sister Sally is one of two nuns who runs Saint Albert's Orphanage for Boys. She was to appear in Casey Jones in Muscle & Faith, a canceled Mirage Studios graphic novel that was later released online by its author.
Sister Sally and her colleague Sister Sue are friends of the Ninja Turtles, and it is not elaborated on how they know them - it's only said that it's a long story - and they are at least familiar enough with the Turtles to know their names, but also Sally calls Leonardo by his nickname, Leo, when referring to him in passing.
When a pair of brothers run away from their orphanage to the Badlands, a section of condemned buildings fenced off from the rest of the city that is run by the amoral Mother Drekk, the nuns go to the Turtles for help. However, they have the flu at the time, but direct them to Casey instead.
The nuns break into Casey's apartment (with Sue picking the lock), and lay in wait for him, surprising him when he returns home. They pitch their case to him, and he agrees to help.
The following night, Casey and Sally set out (Sue, despite having her heart set on the mission and even dressing to infiltrate, is ordered to stay behind with the orphanage's bus). They slip into the Badlands and come across three drunken children - members of Drekk's gang, the Badlads. They intimidate the lads into giving up the initiation location - the old Rialto Theater. Once there, Casey and Sally witness Dustin and Ryan - the brothers - among those to be initiated. Casey dresses up as a hillbilly and distracts the Badlads with a terrible banjo solo, while Sally tries to rush the brothers out. Mother Drekk, unfortunately, recognizes the ruse and has her Badlads attack, then knocks Casey out.
Casey, Sally, Dustin, and Ryan are tied up and made to play "family" with Drekk, but Casey gets loose and fights Drekk again. Ratface, Drekk's number two, tosses her a shotgun, and against Sally's warnings, the brothers attack Drekk to save Casey, knocking the gun into the surrounding mud. Casey saves the brothers from Drekk's wrath, and she picks up the shotgun, but the mud causes it to misfire and blast in her face, killing her. The Badlads now realize they're free, having become disillusioned with Drekk's way of life, and Sister Sally convinces them to come to the orphanage.