Having a conversation this weekend, and this came up. Naively, I'd be inclined to simply label the brother of my brother-in-law a brother-in-law as well, since we've joined the families in a sense.
Specifically, my wife's sister's husband has two sisters. I call him my b.i.l., would they be s.i.l. by extension? Is this acceptable usage, or must I trod laboriously through the above description every time I describe the relationship?