Girls: If you were her, what would you want?

Ok so I guess I'll give a little background info. My best friend Hazel and I have been close friends since about age 6. We met at a hospital because we've both had health problems throughout our lives. We've been through a lot together, and since high school we've been inseparable. She was recently re-diagnosed with cancer. So as you can imagine she's in a lot of stress right now. But I just recently realized that I really like her, more than just a friend. I'll do anything to keep her alive, and I'm hoping that they'll accept me as a possible bone marrow donor for her. I'm afraid that if I tell her that I like her, she'll just think I'm joking (We joke about stupid things like that all the time-- we're married on facebook :P). Either that or it'll ruin our friendship. So my question is: If you were Hazel, and you had just started cancer treatment, would your best friend telling you that he likes you push you over the edge? I just don't want it to be too much for her. I don't know if she likes me back. She never talks about who she likes. Thanks.