This is a word that most native speakers of English do not understand. “Co-son-in-law” is a term that we Indians have contributed to the English language. It is not found in many dictionaries. When you refer to a man as a “co-son-in-law” what you mean is that the two of you have the same in-laws. In other words, this other individual is married to your wife’s sister. Native speakers of English would refer to the other individual as a “brother in law”. Most Indian languages have a term for such a relationship, and I guess that’s the reason why we felt compelled to come up with “co-son-in-law”.