Thanks for your reply! DP2 is not concerned about the term. We're all far more pragmatic than that.
In my years of poly, this is the first time, though, that I've had to deal with the situation of having one 'legal' partner, and one....I guess 'stranger under the law' partner. Until recently, even though I'd been with DP1 for literally decades, we had no legal relationship. Now, thanks to the changing political climate, I have been allowed to place a legal status on my relationship with DP1. Which, by default, makes me even less able to place a legal status for DP2.
I guess I was 1) venting my (mild) frustration of not being able to choose a 'status' that honors the reality of my family, and 2) checking in with folks to air my suspicion that choosing the closest semblance available (sister-in-law) might be unwise because it would not stand in a court of law, irregardless of how we view our family structure.