Question

Pointing to a boy a women says,"his father is father-in-law of that person whose father is father-in-law of mine". then how is the boy related to the woman?

Answer 1

as you said in your question , so my answer to your question us that the boy is the brother of that women or girl as it seems they both are brother and sister

Answer 2

Here, the boy and woman are brother and sister. The boy's father is the father-in-law of the sister's (woman)  husband (person). Then, the husband's father would be the father-in-law of the sister.