As the turtles was walking home after their mission as Leo saw a shell laying on the streets

Leo: guys look

Donnie examined the shell as it pop out a turtle little girl

They all was shocked

Mikey: oh poor thing she's just a child

As splinter recognize her

Splinter : nikki sweetheart Is that you?

As she looked at them as the turtles looked confused

Mikey :uh pops you know her

Splinter noded: she's your long lost sister