only you can answer that question.
ANSWER:
They aren't really considered your relatives, but they are your daughter's mother in law and father in law.
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