Where to start? First, the Schindlers can always bring move a court to reconsider a decision based upon new evidence, and they have done so. The courts have been uniformly unmoved by this new evidence. In short, they either didn't think the evidence was relevant or dismissed its credibility.
Second, a federal court is constrained to hear only cases arising under the Constitution and laws of the U.S. Congress. That is the extent of the jurisdiction of U.S. courts under the Constitution, and Congress can only expand their jurisdiction to this extent. Congress does not have Constitutional authority to pass laws dealing with probate matters in the like. This is a general police power type of law that only states may pass. The Constitution does provide the power to ensure that persons are not deprived of life, liberty or property by the government without due process of the law. That is the only kind of case that a federal court could hear in this matter, regardless of what Congress might have hoped they do. In order to determine whether a person has received due process, the court looks to determine whether they (in this case, either Terri or the Schindlers) have had a full and fair opportunity to present their case before an impartial decision-maker. It is clear from the record of litigation that they have, and the federal courts have found precisely this. A federal court cannot make a factual determination on what Terri's wishes actually are. That is not the type of case the Constitution allows, and it has nonetheless already been decided in the state courts of Florida.