You are going to respect your father in law. Not the rest of the family. You might be surprised to find out that many people will be comforted by your presence. This isn’t a time to worry about appearances. You don’t get a second chance to pay your respects in this way. As someone said above me, go. Keep a low profile. Offer your condolences where appropriate. You don’t want to spend the rest of your life wondering if you should have.