Please help me, my sister has cancer?

hi, sorry i didnt know what to put for the question. anyways, im 14 years old and in 8th grade. 2 years ago february 9, 2009, my big sister got cancer (leukemia). of course no one was in the best mood but at the time it wasnt the worst. she got to come home some days then would have to go back to the hospital for another week or so. it increasingly got worse and not too long ago she had to be transfered to a different hospital because she went into septic shock or (critical condition) that maybe lasted a few weeks and at that time she had a 40% chance of surviving. she got a little better and was expecting to get out of the hospital in may and her cancer was almost gone, which was great! i was so excited! then one day my dad told me that for the third time her cancer came back and in order to survive she would have to go through a large dose of chemo therapy then the stem cell transplant. unfortunatley, the doctor said that might not even work and now the doctor is waiting forever to start chemo even though its the perfect timing, (when my sister is doing a little better). as of now she has about a 20% chance. i am kind of afraid that my sister just wants to give up and isnt telling us because whenever i see her she usually says shes sorry and doesnt like how we go through so much trouble for her and spend so much money, but i tell her thats not true. honestly, its kind of hard to even make it through school everyday thinking that something could happen to her. and whenever my friends or teachers try and talk to me about it i just break down and start crying. i cant just let my big sister die like that i have looked up to her for the longest time and we have so many great memories together and im starting to think both me and my mom are getting depressed. by the way i have another brother and sister and a brother in law . this is really hard for all of us. it kind of seems like her doctor knows she wont make it but isnt telling us but she gave my sister the option of taking a ton of drugs so it will be a painless death. i really cant let her do this! i know she is going through a lot of pain but i really dont know what to do anymore i need her! she means the world to me! please help me! sorry for writing so much.