Curlee,
I commend you for doing this great and difficult karma-forward act by helping your sister, but from my own experience the depression must be dealt with. Now. It affected (and still creeps into) EVERY aspect of life. My healing and quality of life is definitely affected by my "inside job". Seek a qualified doc that she has a good rapport with-I've had to go through a few so far but being able to communicate openly and honestly is, to me, a major factor in actually seeing results. Depression is a continually recurring obstacle that can rob and undo much hard won effort very quickly.
I don't want to harp on it, but don't ignore your own feelings, either. If you "want to cry" all the time, it is of course, only human-and we all need release from time to time; but I find that I respond much more positively to my own or someone else's habitually positive frame of mind.
Much love, many prayers, and positive strength
Gordon