I know what this is like too. It is really really hard when those we are closest to disagree with what we know is right.
You have to hang in there.
You are doing what is right and even if you fail you will have done everything you can, you won't have failed Mia. Sometimes we do end up fighting on what seems to be the losing side but that is when it is most important that we hang on to what we know is right.
I've have something similar with Gus where I know there is something wrong with him and all the vocal members of my family either disagree or abdicate responsibility. I do have one silent supporter but that is not always much help as I insist on taking a dog they say is well back to the vets time after time. Even when the vet finds him medication that they don't actively disapprove of they still don't want him to have it
.