My brother married his high school sweetheart who has five siblings who all have kids that have grown up calling me aunt. What relation would my sister in law's nieces and nephews be to me?
Considering that the proper term for your spouse's brother or sister would be your co-brother-in-law or co-sister-in-law and though this term would be highly uncommon, it could be suggested that their kids would be your co-niece-in-laws and co-nephew-in-laws. However, the term co-sibling-in-law is believed to have stemmed from translations of South Indian languages, which could be the reason for its wordiness.
protected by tchrist♦ Apr 18 '16 at 5:26
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