One idea could be that the little human sister discovers the Tmnt's lair by accident. Maybe she was exploring the sewers for a school project on urban infrastructure. Once there, she befriends the turtles and helps them out with some human - world tasks like getting supplies or finding information on their latest enemy.
One great tmnt human high school fanfiction is 'The Turtles in High School Chaos'. It has an interesting plot where the turtles try to blend in with human students while also dealing with their ninja duties.
It often explores themes of family in a new way. Since she's their sister, it's not just about friendship or alliance. There's a sense of loyalty and protection that comes with the family bond. Also, the human sister can bring a different perspective to the turtles' battles and adventures, adding more depth to the story.
The unique part is often the blend of two different worlds. The turtles' ninja and mutant nature against the backdrop of a normal human high school. It's like seeing aliens in a regular school.
One great 'tmnt human sister fanfiction' could be 'Sisterly Bonds'. It focuses on the unique relationship between the human sister and the turtles, with a lot of heartwarming moments as they face various challenges together in New York City.
Give her a distinct personality. For example, she could be really curious and adventurous, which is how she gets involved with the Tmnt in the first place.
In the traditional TMNT universe, male turtles like Leo getting pregnant goes against the biological norms of the franchise. However, in fanfiction, anything can be reimagined. Some fanfiction might use magic or some out - of - the - box concepts to make such a situation possible, but it's not part of the official TMNT lore.
In this fanfiction, the TMNT might escape by first observing their captors' routines. If the Shredder has minions guarding them, they could wait for a shift change or a moment of inattention. Donatello might be able to hack into any security systems if there are any. Also, they could use their knowledge of the sewers to find a way out. Maybe there's a hidden passage that the Shredder doesn't know about. And of course, their ninja skills would come in handy for sneaking past guards and making a quick getaway.