I am 48 and one of the youngest in our treatment center being treated. So I wanted you to know that I can relate to that.(lol)
But what I really wanted to reply to was what you stated about not wanting to "break down" in front of your sister.
My wife and I have found it so much easier to allow each other to cry or grieve whenever we feel the need and sometimes we even feel the need together. This crazy disease is not just about pain and treatment. It is also VERY MUCH about feelings and emotions and EVERYONE affected MUST be allowed to deal with it in whatever way they deem necessary. ...(safely)When I asked my youngest son(17), if he would go to chemo with me one morning, he said, "No".
I said, "don't you care about my treatment?" He replied, "Of course I do, but I don't want it shoved in my face! What are you going to do next, put post-its up all over the house?" After I left the house I realized, that was just his way of dealing with it all. Then I later heard that he had broken down in front of his friends. We ALL MUST DEAL WITH IT AS NEEDED. Maybe you and your sister should start by discussing the emotions of it. It might just help. I Hope So. May the hand of God be upon you!