How do you call your sister's son's son?

Answer 1

Your sister's son' son is her grandson. Your sister's grandson is your great nephew. You are his great uncle or great aunt.

Answer 2

Shout out their name and if they here you they'll probably answer

or you could call them on their phone

Answer 3

In most families they would call you aunt or uncle. It would technically be great aunt or great uncle.

Answer 4

A nephew-In-Law.