Sia,
It's tough when family members are doing their best to help and the patient isn't fighting for herself, or being cooperative with doctors and family. You aren't alone, because I've heard about this problem from other caregivers, but that doesn't make it any easier for you.
Taste buds can improve over time, but your mother needs proper nutrition right now. You might find useful tips on the web (
here's an example). I'm surprised that TPN doesn't keep her weight up. My wife had TPN after her transplant, and it was essentially the liquid form of eating a proper diet. But maybe that's not the same as what your mother is getting. Has the doctor explained what's going on? Has your mother been vomiting?
There can be many reasons for actions and attitudes like your mother's. Every transplant case is different and every family's interpersonal relations are different. Some parents can't adjust to the reversal of roles when their children are taking care of them. Some can't get past their symptoms to imagine their own improvement. Some are depressed. Some have bad hospital or doctor experiences and give up rather than stand up for themselves. Some feel guilty for putting the family through their illness, and don't know how to express it. I'm a l layperson, not a psychologist, but I know that factors like these can play into the problem.
Perhaps saying the right things to your mother could get her to be more cooperative, but you may need some outside help. The hospital may have counselors, social service workers, ombudsmen, or other people who help with these types of problems. It may be wise to tell your mother's doctor what you've told us, so he/she doesn't think the treatment is progressing smoothly. Also, there are independent professionals who understand how to help families coping with major illnesses. Perhaps you can get a referral. Professionals who "have seen it all" are most likely to have strategies and advice to offer. After all, it's not surprising that you feel lost about what to do, since you've never faced something like this before. Seeking help for your mother shows how much you care.
P.S. to DanL: The patient is Sia's mother, not her sister.