Living donors are so amazing to me! God bless you for even considering being your sister's donor. You represent the best of humanity, selfless, giving and self-sacrificing. After receiving a kidney from a living donor myself I have enjoyed a remarkably better quality of life and have avoided dialysis. Yay living donors!!!
I hope your upcoming surgery goes well, it sounds like you are choosing this option out of self love and wanting to live a healthier life. Good for you, that takes more courage than many people know.
From what I understand donation selection takes into account the whole person. Things like blood type, blood pressure, diabetes, kidney and liver function, life style, financial stability, mental and social factors all play a roles. Many people who fall outside of typical ranges have qualified to donate. Less than 2 weeks ago I saw an article about a man who lost 175lbs to qualify for his sister. I've also read about people who generally considered "too old" are in such great shape that they qualify. I think tireless pursuit is sometimes needed. If you hear "no", ask "why"!