If Jane and John are married, and Jane has a sister, Jane's sister is John's sister-in-law. Jane's sister's husband, however, is not related to John, although he is ''Jane's'' brother-in-law.
These people are part of your extended family. They are not related by blood to you and there is no simple word for the relationship. They are your husband's sister-in-law and brother-in-law, of...