I'm terrified

I'm terrified. My sister was just recently diagnosed with triple negative breast cancer. My question is how to you cope knowing that a loved one has cancer? I don't want to accept it. Spending time with her is so hard for me as the whole time I am with her all I think about is how much more time do we have together? My sister is so young to be going through this. She is only 22. My sister has been through so much in her short time of life. She has battled stage 4 endometriosis and had 2 surgeries for it. She got pregnant with hetr 2nd child, but ended up miscarrying (2nd miscarriage) which resulted in her losing a fallopian tube and 1/3 of her uterus. She can't have kids no more. Then on top of it she recently just left an abusive relationship and is starting fresh. I'm angry with God and I know I shouldn't be, but I cant understand why God would put someone through so much who's already been through he'll and back. I feel like God is taken everyone away from me. My mom has been battling brain cancer all my life, multiple surgeries, hospital visits and so on and now my sister. I know they can leave me at any given time. My sister has surgery this Wednesday to remove a large mass/tumor, but with how advanced it has become (in her lymph nodes) they said they're most likely sure its stage 4 and give her an estimate of 5 years. My sister is so young. She hasn't even lived her life. She has a beautiful 2 year old, my niece that she wasnt even supposed to become pregnant with, but God blessed her. She needs her mommy. I'm sorry if I sound like a baby I just don't know how to cope with it. She acts like she is fine and tells us she doesnt want us to look at her like she is sick and to be grateful that she is still with us as of today, but I still can't bring myself to accept it. I'm sorry I just needed to rant. Her surgery is becoming closer and I'm terrified for my little sister.