Locks are sometimes the only guarantee of protecting one's boundaries. In the case of the creepy neighbor, it's absolute common sense. It may be inconvenient to have to lock and unlock the door yourself 10 times daily, but that is a small price to pay for piece of mind.
With regard to the father-in-law, this is a major boundary issue. He has to be made to understand that he can't just walk in whenever he pleases. Adult children should be respected as adults. If he is the person who housesits when the poster is on vacation, maybe it's time to get someone else to do it who won't barge in whenever he bloody well pleases.