Your nephew-in-law. Just like your sister's husband is your brother-in-law, i.e. has the same type of relationship to you (except for being male) that your sister has, only it is "by law" (marriage) not "by blood" (Biology). Interestingly, your brother-in-law's son would also be your nephew-in-law, and your brother-in-law's daughter's husband would be too.
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