Oh, I am sorry for your loss and the weight you bear caring for your mom. There is no greater privilege than that of overseeing the care for those we love so dearly. I say this as I have born that privilege and weight myself. We must keep in mind this:
There is:

A time to be born,
and a time to die;
Ecclesiastes 3:2

Our omnipotent Almighty, only wise God has ordained the life and death of each person. As much as we might tend to second guess our decisions, like should we have done this or that, would it really have made the necessary difference in extending a person’s life? We can have this assurance, that nothing happens apart from what God has ordained to happen. All people die. And those who remain, mourn, yet we are to balance our mourning with the realization of the following:

“Always remember that, however soon and unexpected your parting with your relations was, yet your lease was expired before you lost them, and you enjoyed them every moment of the time that God intended them for you…Oh, if this had been done, or that omitted; had it not been for such miscarriages and oversights, my dear husband, wife, or child had been alive at this day! No, no, the Lord’s time was fully come, and all things concurred and fell in together to bring about the pleasure of His will. Let that satisfy you: had the ablest physicians in the world been there, or had they that were prescribed another course, as it is now so it would have been when they had done all…He who appointed the seasons of the year appointed the seasons of our comfort in our relations; and as those seasons cannot be altered, no more can these. All the course of providence is guided by an unalterable decree; what falls out casually to our apprehension yet falls out necessarily in respect of God’s appointment.”
~ John Flavel

Consider also that the organs your sister donated may have saved someone else's life. Sometimes we may learn of how they were used. I hope you find some comfort in these thoughts.

The Chestnut Mare