What is the relationship between me and my co-sister-in-law's daughter?

Answer 1

The English language does not include a relationship called "co-sister-in-law" or one called "co-sister." If you are living in another culture and are trying to describe a relationship different from that of basis "sister-in-law," you will need to be more specific.

If your sister-in-law is the wife of your brother, and her daughter is also your brother's daughter, then you are the daughter's aunt or uncle and she is your niece. If she is not your brother's daughter, then you are not related to her. But you can treat her as your niece if you want to.

If your sister-in-law is the sister of your spouse, then her daughter is your spouse's niece and yours as well since in English we talk of "aunt and uncle" not "aunt-in-law and uncle" or "aunt and uncle-in-law."