Probably your niece-in-law.
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Your nan's sister's son would be your first cousin once removed. This relationship indicates that there is a one-generation difference between you and your cousin. In genealogy, cousins are classified by the number of generations they are removed from a common ancestor, with "removed" indicating the difference in generations between two relatives.
If by "your nan's sister's son" you mean "your mother's sister's son", he is your 1st cousin.
However, "Nan" is often short for Nanna, meaning grandmother. Your grandmother's sister's son is your parent's first cousin and your first cousin, once removed.
Well if she is your father's sister then that makes her your Aunt. Then if your Aunt has kids that makes her kids your cousins.