Have you thought about this long-term and long-term earning prospects?
For example, supposing your DD is training to be hairdresser as an apprentice. What is her long-term earning potential going to be?
I don't know what the average salary for a hairdresser is but it's not that high.
If your DS is studying something like Law, he is far more likely to earn a much higher wage than your DD long-term. Solicitors earn more than hairdressers.
Additionally, if your DD was ever to have children, the chances are she'd need to take a career break and will have her earnings dented.
Your DS, blessed as the male sex, may well not have that holding him back and he will continue to work.
I'd look at this as a 10/20 year thing.
I think the other thing is you must offer to DD that if she ever wanted to go to university, you'd be willing to fund her too.
It's not fair that one child should get all the benefits. I have a completely different view to you. I think your DS is actually the winner here, your DD is penalised for staying at home. Awful.
If it was me personally, I'd not be taking money from my DD. If she's getting £1000 per month, she'd be expected to save at least 50% of that if not more. That's £100 per week spending money left over. That money is for a car, for a house, for something special. Teach her to save, don't save it for her, yourself. Teach her to be responsible.
I'd suggest she take out cash ISA - even a stocks and shares ISA and put in the FTSE 100 every month, £500 per month. That's what I'm doing for my kids (not that much money sadly!), they're younger than yours but if they can't earn a big wage, at least they'll have long term stocks etc and it's all tax free.
Teach her to grow her money. And your DS too when the time comes.
And yes, offer her 3 years of extra funded education somewhere too, if she should want it in the future.