Your Sister In Laws parents will be your Mother and Father In Law.
The Yiddish term for "son's in-laws" is "shvere" for the parents of one's daughter-in-law and "makhuten" for the parents of one's son-in-law.
your sister and brother in-laws
There is no genealogical relationship. He is just your son-in-law's brother.
the in-laws should not be in your business like that anyway. a relationship is you and the other person.. not you them and their parents.. if they cheated on you than there is nothing to protect.. he/she violated the trust in the marriage, there should be no excuses.
You don't. This is not your business. But if you have a good relationship with the son, then you can encourage him to tell his parents.
Generally, they would be referred to as "in-laws" as a generic term. Or, alternately, as "our daughter's in-laws". In person, you would introduce them as "our son-in-law's parents".
There is no actual blood relationship. You would typically introduce her in that way, My son-in-laws's sister. Many refer to her as the sister-in-law.
Whether it is common for a son-in-law to be included in the in-law's will depends on his relationship to his in-laws. It is likely that he be included if they get along really well.
my son's mother-in-law = la suegra de mi hijo(there is no single word for this relationship)
You aren't. You do not share a common ancestor.
its so annoying always telling you wat to do