Difficult situation - and hard question. Tube feeding could be considered an extraordinary measure. When I went with my mom (before she died) to do her living will, we included hydration in the care to be given. My mom passed away very peacefully in her sleep so we did not have to face any of those terrible decisions. Still, most of my experience with local hospital people indicates to me they are operating within good parameters - not doing something positive to hasten the death, but not being so aggressive in treatments which would prolong the life of a person who, before the days of all our medical technology, would have died peacefully at home.
I know I am just talking and not giving a precise answer. You would probably have to consult with a local priest about the specifics of your mother in law.