I cannot cite the case, but I heard testimony before a state committee in 2003 that the Supreme Court has ruled that you must presume the parents are acting in the best interestes of their child unless you have evidence to the contrary.
The subject of the testimony was home education, not the Schaivo case. I do not know if the statement applies once the child is no longer a minor.
Shalom.
I serve as an on-call attorney for the Home School Legal Defense Association. The presumption in favor of children exist as long as the child is a minor or a dependent. They certainly do not exist when the child marries. At that point they are clearly emancipated.