An advocate is defined as, "One that supports or promotes the interests of another." Given this definition, as a family caregiver, you are the ultimate health advocate, because you are constantly supporting the health interests of your loved one.
While there's a fine line between being involved and being overbearing, it's necessary for you, as a family caregiver, to foster open lines of communication -- and this includes questioning decisions concerning your loved one's health.
Without all the people who find themselves caring for those with terminal illnesses, our healthcare system would be overwhelmed. Yet we as a society do little to support them. There is much more we can do.