As in: you're not really just getting one person, you're getting their parents, siblings, extended family, friends, etc etc etc.
So what do you do if you hate an in law? Not that I'm married or even engaged, but I was thinking about an ex whose mom used to drive me BONKERS. She was always trying to give me her old clothes (which A. didn't fit me and B. weren't my taste) and I was always trying to make excuses why I couldn't take them. Ugh!