The question as posted reads:
Is there a term for the sibling of ones son-in-law or daughter-in-law? Real-life example: The composer Dohnanyi had a son who married a sister of Dietrich Bonhoeffer. So Dohnanyi was the father of Bonhoeffer's brother-in-law, or the father-in-law of Bonhoeffer's sister. Is there an accepted term for this relationship?
To me there's a big difference between the question like the one above, and a question which finds a specific term in a document and asks how it should be interpreted.
The "what do you call it?" question is a question that a field linguist would ask an informant. The "what does senior/junior mean in this context?" question is a question a client would ask a genealogist.
Here's a new wrinkle:
Our guidelines say "You should only ask practical, answerable questions based on actual problems that you face."
The scenario in the question -- determining what the relationship is -- may be a problem that needs to be solved 100 years from now, but is it a problem now? This is a legal/social question. Is it a question for us? The genealogical relationship between all the parties does not change because one of the family members dies. The wife's parents are still the grandparents of the grandparents; the husband is still the son-in-law of the wife's parents for the date range they were married. The only thing that changes as far as the genealogical record is concerned is that the date range for a marriage has an end date, due to one of the parties being deceased.