I'm in favor of clarity whenever more clarity can be conveniently mentioned, so I'd tend to say, "This is my sister-in-law Sandra, my younger brother Bob's wife" or "...my sister-in-law Lily, my husband's older sister." I kind of agree with what others have posted about one introduction being less friendly than another, but I would tend to go more toward familiarity and explanation than otherwise because I have a lot of good, otherwise close friends that introduce me to people in rather a vacuum: "Oh, yeah, this is my brother." Well, that's nice, what should I call him, should I want to? Which of the brothers might this be?
Or I get, "Hey, I know you'll have tons in common, you both [whatever interest] so I can't wait for you to meet Karen!" - and Karen might be whom? Friend, stepmom, half-sister, co-worker, sister-in-law? In a way I don't need to know 100 years of family dynamics and changes and so in a way it doesn't matter, but while 'Karen' might be perfectly nice and friendly, it's nice to know where she fits into the overall family /friend framework. That way, one minimizes the chance of inadvertent mis-step or insult.