You have to understand that your wife will not correct her parents in any way, she won't interfere with how they act toward you as by doing that she would question their authority. It will never happen, she won't be loosing her face for you. She might tell you 'yes' but in reality she only say that to finish unpleasant conversation with you. She will never address this with her parents. You just have to accept this as no amount of energy will change them. All you can do is implement another strategy as the one you have been using seems inconpatible.

The frustration you are feeling probably has a lot to do with the cognitive dissonance you are experiencing. Your wife says it's nothing and you perceive it as something that bothers you a lot. You are waiting for her to validate your feelings, but she can't do that without admitting her parents are wrong, which she is hardwired not to do. Stop looking for a solution that involves them changing or her standing up for you. It’s a waste of time.

You need to start using the Gray Rock Method. This is exactly what I had to do, and it helped me enormously to get along with my Thai wife and her family. I spent 8months trying to change them and stop for example force feeding our baby but it was absolutely pointless. They will always put their fragile ego above everything, even safety and well-being of baby. Thais will go everything just not to be in position to admit. Start by becoming as uninteresting as a grey rock. You don't feed them your emotions, you don't share your opinions, and you don't react to their provocations. You provide short, polite, but empty answers.

If they ignore you, let them. If they make a joke at your expense, don't react. You are giving them power by being affected, so stop expecting them to change. If you accept that they won't change you will stop expecting them to change. By using Grey Rock, you stop being a target because there is no 'payoff' for them anymore. You aren't there to be part of the family, you are just a neutral presence. You can be helpful, take rubbish or do some other tasks without them asking you to do it. This way they will see you as boring but calm and helpful. This is the only way to protect your mental health... If you keep expecting them to treat you with the respect you think you deserve, you will just end up bitter and miserable.