- I have a sister named Charlotte.
- Charlotte is married to Martin.
- Martin has a brother named William.
Martin is my brother-in-law, but what is William to me?
As you have been told on Genealogy & Family History, there is no "technical term" for the relationship you describe.
In most areas of language you select the words used to suit the purpose you wish to achieve. If my sister's husband has two brothers, then