Spend time with her. Schedule some "dates" with your daughter and give her all your focus. No 7-year old brother who had all of your attention for a year (and longer, since the "C Word" is always hovering in the background).
Consider the pain that cancer brings to a family dynamic and that your daughter, though she doesn't fully understand the root of her emotions, likely resents the pain and trauma her family has gone through due to her brother's illness. What was the family like before cancer? She can't have that back, and in her mind, it's his fault. Ouch.
At a minimum, just go and sit with her for 20 minutes every day. Ask about whatever she's doing or watching at the time. Don't make it awkward. Don't try to have some deep conversation, just BE with her, doing what she's doing, showing interest in her interests, and letting her know (without saying it) that you love and accept her just the way she is. She is a special person and a member of the family. She is needed and loved. Like I said, don't overdo it, or she'll be rolling her eyes and pull away. But DO spend that time with her....and pay attention when she's seeking your approval and acceptance in little ways. The number one thing that kids need is to know that they are accepted and they don't have to DO or BE anything other than exactly who they are. They don't have to "earn" your love.
I hope this helps.
Also, please do consider carefully searching for a counselor for her that she'll actually like. Sit down and talk with her candidly about finding someone she can talk with. Someone who isn't going to talk with you, but will help her to work out some of the anger and hurt she is feeling. Not because you think there's something wrong with her, but because you both want her to be happy. Let her know that whomever she talks to, if she decides that counselor doesn't click with her personality-wise, you'll find her a different one. Show her that you'll go the distance to help her heal what's hurting.
Love to your family at this time. God bless.