That depends on whose gathering it is, doesn’t it? If a couple hosts a dinner, they might well invite the siblings of both of them, in which case you would be at dinner with your sibling’s spouse’s siblings. And if the siblings on both sides bring their spouses, as is entirely reasonable, you might even find yourself at dinner with your spouse’s sibling’s spouse’s sibling’s spouse. Though I’ll grant that you’re unlikely to stretch it any further than that.
You make a fair point, Chronos. I certainly wouldn’t call you wrong for referring to a more distant relation at such a gathering as a sibling-in-law. However, I suspect my suggestion describes the conventional limits.