Lets be completely honest here - your grandmother has done better than 90% of the population. Traditional medicine probably would be too agreessive (ie chemo) and in all honesty might not even improve things, especially at that age. I really don't see the harm in the homeopathic route, especially if this is what she wants. Its not "spiritistic" by any stretch of the imagination. Hell ALL pharm drugs come from plants of some nature. Remember one thing, when you get that old, would you want to be told how your final days would be? Give your grandmother what she wants and bring the home care person back in. Maybe she will see that you have always cared about her, no matter if you are a dub or not. I would not let your younger sister dictate how your grandmothers last days will be on Earth. In fact the stress added to the situation has probably done more bad than good, no matter what your sisters intentions were/are.
Also, if your grandmother is going to be buried I would hold your own grave side gathering where you can truly express yourself to those present and let others do likewise. The JW services are a load of BS and I would not allow that to conclude the matter - honor your grandmother both in life and death and think about what you would want others to do. If she is being cremated, you can talk to the funeral home performing those services and they will normally give you a parlor/room at no cost, or inexpensive where you can hold a small eulogy. Invite all your family, her friends, neighbors, etc. and memorialize her, not by religion or Bible, but as a person who lived and shaped lives.