Hi everyone,
My great-grandfather was Giovanni Di Stefano, born around 1887, possibly in or near Naples (Napoli). He immigrated to the U.S. around 1910, and I believe he entered through New York (Ellis Island Slope Game ). I’m not sure of the exact town he came from. His wife’s name was Maria Russo ...

Your sister gets married.
Neither she nor you are blood related to her husband. You both are merely marriage related to her husband.
The couple has a child.
The child is blood related to his/her mother and father, so therefore "logically" both you and your sister are now blood related to the ...