Can the son of the divorced woman's sister be an agent for the young man in the marriage contract?

It is not permissible for your sister's son to marry you off, according to the majority of scholars. Rather, your closest guardian (wali) should marry you off (the grandfather, then the son, then the full brother, then the paternal half-brother, then the nephew, then the paternal uncle). It is permissible for the groom to appoint someone else to accept [the marriage], just as it is permissible for your guardian to appoint your sister's son or someone else to marry you off if it is difficult for the groom to meet with the closest guardian.