Hi twaflooers, sorry to hear of your sister's condition.
I feel you were put on the spot being asked in such a way, plus, you also have the emotional side of the situation with it being your sister.
I can fully understand your reasoning as to why you wish to keep both your kidneys and think that as long as you remain honest with her you shouldn't feel guilty in any way, shape or form.
However, If it were me in that situation I'd be looking where to sign. I know you have children and are worried that they might develop the condition, but that's a big might.. your sister is here and now and the problem is real.
Either way, I'm sure you'll not take this lightly and will think about it in great detail before making your decision.
As I said, you have nothing to feel guilty about, just be honest with her.
Good luck :)