I think you are going a little far out on a limb with both those statements.
While I applaud your sense of duty to your fellow man, I would consider it wise to temper it with a sense of duty to your fellow men.
Consider this, the fellow you don't know, may be the social equivalent of Joe Stalin, Mao, Fidel, Arafat, Ahmadinejad, ect. One who's lifespan is measured in the deaths and misery of countless innocent humans. You could save more lives by denying your organs.
As for the sister as a sociopath, based on what?, that she might have more knowledge of her brother than you? A little harsh considering what defines a sociopath.
I think that might have been touched on in the write up but then that's just me.
If he is a monster, he should live to stand trial so, either way, saving his life makes sense.
As to his monster sister, I think I am being generous in calling her a sociopath, heartless f@#$ing bitch is more like it. Remember, she initially agreed to donate so he couldn't be that bad.