It can be said that parents and other family members are too emotional and too closely involved to be the best advocates.
On the other hand, it is precisely because they are closely and personally involved that parents can be or become good advocates.
In many cases they have the most at stake and they have the sense of urgency needed to ask for the change to happen. And they can identify with other parents, because they have "been there".
Parents may require training and specific directions. But when armed with the necessary skills, they often become outstanding advocates.