Its simply what they are with "in-law" added to the end i.e. son-in-law, mother-in-law, etc.
Technically, there is no relationship. You do not share a common ancestor. However, you may share common descendants.
She is the co-mother in law of you and your daughter's father in law is your co father in law. :-)
Mother-in-lax & Son-in-law - i think it is correct
Stepson; Son in law; Brother?
Your mother's son is you or your brother. Your mother's son's daughter is your daughter, or your niece. Your mother's son's daughter's mother is your wife, or your sister-in-law.
I would think you would call her by her name, same as if she were your son's American mother-in-law.
If your sister-in-law is the wife of your brother, she is the mother of your brother's son. Her son-in-law would then be the brother-in-law of your brother's son. If your sister-in-law is your spouse's sister, her son-in-law is not related to you or to your brother's son.
The Mother-In-Law, or Mother and Father-In- Law together, but Mother first.
No because you are not involved in the marriage. Your husband's or wife's sister would be your sister-in-law. Your son's mother in law is not related to you in any way.
my son's mother-in-law = la suegra de mi hijo(there is no single word for this relationship)
You are not related to the parents of your son-in-law, although they are your son's parents-in-law. You share no common ancestor, however, you may share a common descendant.