I am so sorry. I have lost my mother, her sister and her father to cancer. I have recently lost my husband to cancer... The pain is huge... I'll share with you the only thing someone told me, that helped me a bit as it made sense : 

Life is unfair but it is how it goes. The ony thing we can do, beside helping with kind words and gestures, is to spend time with those we love.

Take care of yourself