Full body image of Lavina
She's a red eared slider like Leo and is from the same clutch of eggs as Leo. In my au she would not have grown up with the turtles, she would've been found by Big Mama and forced to fight in the Nexus.
Leo and Donnie are still twins, but they've always felt as if something was missing. (Leo moreso than Donnie) After the events of the movie the hole would be much more noticeable to them.
Lavina, always knowing that somewhere out there was her brother, managed to escape Big Mama and desperate to make sure her bale was okay (because the events of the movie severed Leo's connection for a bit, I will die on this hill).
So when the twins eventually find a turtle who looks like Leo and who seems to find them familiar they decide to bring their sister back home. (The three of them together would be called the Terror Triplets)