After the turtles and master splinter got home from the battle nexus

Michelangelo: uh dudes whats that? He pointed to a glowing shell

As Donetello look and expect it : looks like another mutant ninja turtle but its a 5 year old Mutant girl

Splinter then recognized her

Splinter : nikki? Sweetheart is that you?

Leonardo: master splinter you know her?

Splinter nods: she's you're long lost sister

As she hides in her shell