This can be a tough one if you are just starting out learning Japanese, but there are plenty of reasons why you should make that extra effort to speak to your in-laws using the less familiar style of Japanese.
First of all, polite speech is a sign of respect. If you are getting your Japanese from manga (comic books) and anime, you are surely going to be using vocabulary that will not reflect well on your upbringing. (What did you expect?) It might not be the most popular method, but good ol’ fashioned flashcards can be a great way to sit down, study, and make sure you are showing your in-laws the honor they deserve. (Or don’t.)
This is also a great way for your kids to get exposure to the various forms of Japanese. If you’ve already got your friends and are only using the plain form of verbs and lots of slang, you could wind up with kids who are reluctant to learn the fluffy side to Japanese. I remember how one of my native-Japanese host sisters struggled to figure out the various polite forms as a child. It would be so much easier to just get this out of the way early and actively teach your kids to speak politely.
Another reason for maintaining a high level of speech etiquette is your father-in-law. Casual Japanese, unfortunately, has a tendency to break down barriers between you and the other person. This is great if that person is your husband, but you want to keep up as many barriers between yourself and your father-in-law as possible to avoid misunderstandings and unwanted attention. You don’t want people getting the wrong idea and you certainly don’t want to be causing tension between yourself and your mother-in-law for accidental passes at her husband. Keep it professional, ladies.
It might seem strange at first, but it is also nice to use your spouse’s first name followed by -san when you are talking about them or addressing them in front of your in-laws. This is another way of showing your in-laws that you are respectful of the person you chose to marry and will eliminate any awkwardness felt by the silly nickname you chose for your spouse. And we all know how irresistible those names can be!
Japanese is a lot of work and there are thousands of books out there on etiquette related to speech, but just keep doing the best that you can and know that you are not alone in your struggles!
Render therefore to all their dues… honour to whom honour. (Romans 13:7)
Honour thy father and thy mother: that thy days may be long upon the land which the Lord thy God giveth thee. (The 10 Commandments, KJV)