The nine Supreme Court justices hold their offices "during good behavior" according to Article III, Section 1, of the US Constitution. This means that they may hold office for life.  Maybe "good behavior" is the important point there. If she's in a coma, she can't behave at all. "No behaviour" includes "no good behavior".  The other interesting point there is "for life". A public proof of life must be demanded. If she is "just" braindead, this has to be interpreted thoroughly.