My in-laws like to ask about the private matters of our marriage. How can I avoid answering without being rude?

You have the right to keep private matters private without answering every question; a polite boundary is not rudeness. Prepare a short, friendly answer that closes the topic, for example ‘Alhamdulillah, all is well,’ without going into detail. Steer the conversation to general topics comfortable for everyone. If the questions continue, you can say with a smile that it is a matter just between the two of you. Agree with your spouse on what can and cannot be shared, so stories do not leak out from one side. Keeping your household's secrets protects it from interference and other people's interpretations. Keep the relationship warm in other matters. See in-laws.