I agree with Sajid. I think that concealing her illness will put a lot of stress on her.
When I was younger, *I* didn't want anyone to know about my illness, because of a situation that happened to me when I was 6. I was in a popular newspaper, here in New York, and they did a story on me about Cooley's Anemia and insurance issues. There was a HUGE picture of me in it, with my pump on. When I went to school the next day, I was attacked with comments. No one understood what Cooley's Anemia was, so everyone just kept saying that I had "cooties" and many of the kids parents wouldn't even let me hang out with them anymore. So, I vowed not to tell anyone about it after I moved from the city. My family always knew, from the time I was diagnosed, but I didn't want any of my friends to know anymore.
Anyway, when I had to conceal it, it was very hard on me. I couldn't have sleep-overs, and I couldn't sleep over anyone else's house. Consequently, I ended up leaving my pump off a lot, just so that I could feel like a normal person, like everyone else that didn't need to use that darn thing. I had to constantly make up stories as to why I wasn't in school when I got blood and other stuff. It put a lot of stress on me. I eventually ended up telling everyone, and explained in great detail what it is, and why I have it. I realized that the only reason why I had a problem when I was a kid, is because all those ignorant parents didn't take the time to understand what Cooley's Anemia was, and didn't want to sit their kids down and explain it to them either. It all has to do with education. If those parents would have read about Cooley's, and realized that it's a GENETIC disorder, and not something contagious, maybe they wouldn't have made their children ignorant, too.
I know this is a long story, but I learned a lot from it. I learned that my life now is MUCH easier, ever since I started telling everyone about my illness. I started to tell a few close friends at first, but then after high school, I didn't mind who knew about it, as long as they UNDERSTOOD what it was all about. I even lectured about it a bunch of times while I was in college, because I knew more about it then my professors did.
I don't know what I would've done if my family didn't know that I have this illness, because they were always there for me, no matter what. I don't think it would do any good for a child to have his/her illness concealed from their family. It just decreases the amount of support that the child needs from their family to grow into an emotionally healthy individual.
This is entirely my opinion, and I wouldn't judge someone for something that I didn't agree with, so please do not think that I'm putting you down in any way. I just think that it will make things harder. If you are going to conceal the illness from people, I would at least tell your family about it. You'd be amazed at how much more support you'd probably get.