Does my kid sister know I have MS? Do I tell her?

I've got two half-sibs, a 10-year-old brother and a 12-year-old sister, the children of my dad and his second wife. They're great kids, and we see them often. My sister B. is staying with us for a few days. I don't really know what B. and her brother know about my health. I've never talked about it with them, but several months ago I asked my dad if he'd discussed it with them. He said that he hadn't, though the kids understand that I sometimes get tired and that I have some limitations that prevent me from doing certain things.

I think I'd like to talk about MS with B. this weekend. She's a bright, creative, funny, and thoughtful kid, and I feel like it's not fair to either of us to leave her in the dark about MS. On one hand, I can understand how my Dad might be concerned about scaring them or whatever, but on the other hand, it's probably scarier to know only part of the picture. Sooner or later, she's going to meet somebody who has MS or who know somebody who has MS (if she hasn't already) and what if that person with MS is much more severely affected than me? She needs to know that MS isn't going to kill me, that MS is different for everybody, and that if I need to use a wheelchair, I'll still be me, just in a wheelchair.

I see a lot of myself in B. Last night, I went to bed first, so I said good night as I passed the room where she was reading. Then I thought, we should hug, right? So I turned back to her room, and she had gotten up to give me a hug, but then she saw that I might not have been waiting for a hug. Then she saw that I hadn't been waiting for a hug but had changed my mind and turned back for a hug. She gave me a little smile, half embarrassed at our awkward encounter and half entertained by it. We hugged. I said, "I'm really glad you're here," and I meant it.