It's easy to be generous with other people's organs, isn't it, or to sneer at the very idea of not doing this? The reality is that there is no simple answer here. Family dynamics can be so complicated and there is almost always a complex set of relationships and roles within each family.
OP, it does sound as if your sister may not be well enough for a living donation, or indeed for any transplant. But I think you need to hear that from her doctors. There is truly no shame or blame in not taking the risk (if it is 1 in 200 that you would die, then I wouldn't take the risk at all).
As an aside, are your parents reliable? If they agreed to proved help, would they stick to that? My parents are unusually bad in this respect, but I absolutely know that they would agree to it in order to save my sister (the golden child!) and then go back on their word and mutter darkly about how I was responsible for my own children once it was too late for me to back out.
There are people for whom I would do this, but I am not a single parent and my wider family (well, my in-laws) would be very helpful and supportive. In your situation I would have to see it as impossible.
I have enormous sympathy for everyone in this situation, and I understand all too well the all-consuming need to "fix" the problem, but go carefully and be gentle to yourself.