Your sons being "star pupils" at C is irrelevant. But them being pupils there may well push her up the criteria - but that depends on if they give a preference to siblings, which is not universal at secondary, as children are supposed to be able to make their own way to school at that age.
What happens is: all applicants for school A, and all applicants for school B, and... are sorted into a big list according to each school's preference requirements - usually SEN, Medical, siblings, distance but may include religious criteria and feeder schools etc.
Then the these lists go back to the LA, and they work out the cut off point, but then the lists shuffle as some people qualify for more than one school - they get their highest preference of the schools they qualify for. This is all done by computers.
Now what I really suggest you do is find out why your DD doesn't want school B. If she gets offered school B, as well as making sure you are n the waiting list for school A - I would contact school B and see if you can visit and allay her fears. She might well change her mind if someone she gets on with is going there anyway.
And if there is a good reason why A is a better fit for your DD, then you could win a place at appeal.
My DC went to a very highly thought of school. My eldest got in via the waiting list having been 25th or lower to start with. Other children got in every year via appeal.