Let's put this delicately: In-laws are a bit like outlaws. When they show up, they come bearing issues. If you find yourself getting a migraine every time your family visits, a few well-placed hints will keep them out of your living room. If your parents or in-laws think TV is for the devil, then make sure your huge entertainment center is right in front and the surround sound speakers are in every corner. A roll of crime scene tape wrapped around the banisters and strung from the doorknobs is a nice touch, but don't rely on subtlety. Change the locks, and if they do manage to make it through the wreath of garlic you mounted on the door, barricade the bathrooms. (That'll show 'em.)
Having a nice area to entertain may encourage undesirables -- hey, you may love your in-laws but wish your mooch of a brother would forget your address. Whoever your pesky guest happens to be, don't let him get toxic all over your special space. It'll spoil the good vibrations.