"Finally, if I may point out that you seek the definition because you are giving a public address, the smartest thing to do is to refer to your sister-in-law's husband Harry as "Harry, one of my in-laws" and leave it at that. Nice and nonspecific, but undoubtedly true, for Harry is one of your in-laws, let the listener decide which one if they want to press the issue."
Actually, I intend to include a person of rather high prestige in one of my newsletters which I prepare for my Japanese middle school students weekly. This person attended our End-of-Year family gathering and is in the family photo. He is the VP of Meio University, and quite well-known in Japan and South America (Brazil in particular).
I have determined to not introduce him as my brother-in-law, but simply state that the event was attended by Prof. Junji Sumie, VP of Meio University. If asked personally why he attended a simple family gathering, I can then mention that he is also family -- my brother-in-law.