I think it’s time we sort out what kind of siblings Karai and the turtles are to each other.
I see a lot of posts saying that Karai and the turtles are surrogate siblings, adopted siblings, half-siblings, or even step-siblings. I think there’s a lot of confusion as to what these different terms mean, so I thought I’d break it down.
Brother, Sister, Sibling: Someone who shares the same biological mother and father as you. Even if someone is your half, step, foster, or adopted sibling (or just a close friend), you can still forego any adjective and simply call them your brother or sister. Half-Brother, Half-Sister, Half-Sibling: Someone who has the same biological mother but a different biological father as you, or someone who has the same biological father but a different biological mother as you. One example: Your parents divorce when you’re little. Years later, your father remarries, and some time later he and your stepmother have their own baby boy. He is your half-brother… your literal brother from another mother, if you will.
Stepbrother, Stepsister, Step-Sibling: The biological (or even perhaps adopted) child of your stepparent that has no biological relation to you. One example: Your mother marries a man who has been married before. He had a daughter with his first wife. His daughter is your stepsister.
Foster Brother, Foster Sister, Foster Sibling: Someone in the Foster Care system who lives with your family temporarily, indefinitely, or until an adoption is completed. This child is not likely related to you biologically.
Adopted Brother, Adopted Sister, Adopted Sibling: Someone who may or may not be related to you by blood who your parent(s) or guardian(s) has/have taken in to raise. These days, child adoptions in developed nations are legal matters. In the past and still in some places, adoption is an informal agreement. Because an adoptive parent is the one who adopts the child, adoptive can be used to describe the biological child of the adoptive parent in terms of relation to the adopted child.
And for good measure: Surrogate Brother, Surrogate Sister, Surrogate Sibling: Surrogate means substitute, so these terms are used to describe someone who is not actually your brother or sister, but is very much like a brother or sister to you, or fills the role of a brother or sister in your life.
The turtles and Karai are not half-siblings because Splinter did not marry a turtle lady and have four turtle sons with her. The turtles are not biologically Splinter’s. We also don’t know in this series who the last human was to touch the turtles before their mutation. It may have been a pet shop employee. We just don’t know. Even if it were Hamato Yoshi, the turtles wouldn’t have enough of his DNA to be considered Karai’s half-siblings. His DNA wouldn’t replace their biological father’s. After all, April has Kraang DNA but she still has both her parents’ genes as well.
The turtles and Karai are not step-siblings because Splinter is not married to the turtles’ biological mother. That’d be… that’d be a very weird ship.
The turtles and Karai are not foster siblings, because none of them are part of the Foster Care system.
One can argue they’re surrogate siblings, but I think they are a bit more than substitutes to each other.
Splinter adopted the turtles (albeit informally). They are not his biological children, but he decided to raise them as his. Therefore, the turtles are Karai’s adopted brothers, and she, as Splinter’s biological daughter, could be considered their adoptive sister.
Awww, look… they’re just as bored as you are after reading all that. And Leo does not look amused. lol