Please tell her you like her!!
My mom had a breast cancer 3 years ago (she's thankfully fine now) and I was so glad to have my boyfriend at the time to lean on for support.
Talk to her. Tell her you're interested in her. I'm sure you've been supportive for her through her moms ordeal already, and as a boyfriend type that support can be so much more. I can understand that you are worried that at the moment she might not be in the best mindset to start a relationship, but don't wait.
She may or may not know that you have feelings for her. A few months back a good friend of mine told me that he had a crush on me last year and I had no clue at all. Sometimes you just have to come out and tell the other person. We're not all mind readers
Good luck, let us know how it goes.