Any financial input yourself or your family wish to give are for your daughter and not for the other children even necessarily know about, you can't be expected to treat someone else's children the same so don't feel guilty. Your daughter is extremely lucky to have this financial support.
However, I think perhaps a gift of £10k is a little large to be given to a 15 year old, especially if it is destined for her university fees - could this be moved into a high interest account until she reaches university age?
If I was a 15 year old with that much in my bank account, I wouldn't keep quiet about it, particularly if I fell out with my siblings and wanted to use it to antagonise them. She may also tell someone unsavoury, who could try and gets their mitts on the money.
I'm not trying to insinuate that your daughter hasn't got good common sense or is unable to know when people are genuine, but people may change in their behaviour towards her if they know she has what is seen as a lot of cash at that age.
If it was me, given that the money has already been gifted, I would chat to her and explain my concerns, and offer her the option of several savings ideas which would lock at least part (if not most) of the money away for a few years until she can use it wisely.