Tension between people and their in-laws has been around for as long as the phrase itself. No law requires you to spend every other holiday with your in-laws... Or any holiday for that matter. But that doesn't answer the question.
You may think 'in-law' means your spouse’s family is related to you by law, not by blood — yet, in actual fact the usage began as a reminder about which relatives Catholic Church canon law prohibited you from marrying. FULL STORY