How do I navigate this?

First, I want to validate that what you are going through is incredibly difficult due to the complexities of the families we marry into. The family is correct in saying that they have all known each other before you. Even though you come in with a role that is significant to theirs. They are correct in wanting to help and be there for the daughter at all costs. They are not correct in how they are going about it, however. 

In these situations, we ask ourselves, "what is my role here?" We always have a part in how we play in any family dispute. We cannot overlook this, even when their moods are erratic and reactive and their perspectives are misguided. One thought I have when I read this was, "what happened between you and your daughter to the point where she doesn't want to go to the wedding?" You guys have been around each other for years, and now she decides to take a stand? Is there anything that can be discussed about this situation to help the individual resolve her situation on her own? 

So, you are stuck in the family with people on your husband's side who have turned against you. They have revealed their hand in that they will not have your back. It is for these reasons you have to navigate with an awareness of yourself, your emotions, and thoughts, and an awareness of what in life gets your attention causing you to react in a way that could harm your chances of getting what you want. 

I am not sure where your husband stands on this, but is he torn between family, between his child's mom, and also managing your relationship? I am not sure what the people in the girl's life feel about things and if they believe they are actually helping or if they are doing something to be defiant or to try to find a cause in life to justify behaviors? I am leaving you with more questions than answers, but the questions are to "navigate" what's next.  

Navigation starts when we are aware of what we are navigating, practice the use of skills to cope and practice tolerance in understanding how to meet people where they are at. It's not how we navigate it but how we deal with the emotions triggered at work, school, or anywhere. Many specifics are probably not being met as expected here, but there is still a problem that you notice and then there is another way to visit this problem to be more effective. 

I want you to write down what you think the problem is, really is, though. If you start to say you are angry or mad or other people made you upset, that is not the problem. The problem is what interferes with what you want in life or what interferes with things you hold very deeply. The problem is often very much in our minds and our inability to accept our role in situations. We can only control ourselves, so it is critical that we work in the one area we can work on, in us. Look at how you contributed then you can see others in this mess and ask yourself what you can do about them in your life. This isn't easy and is often not a solution to make you feel better, but you will learn to be more effective at getting what you want.