Sorry what?
What age is DD?
Time for mum to start being the grown-up.
You now have contact details of all the parents. Send a group message out to all parents explaining exactly what happened.
You had limited numbers. Your daughter wanted other close friends to attend and went behind your back, inviting other friends, not realising the consequences.
List the children on the original list who are invited. Apologise that daughter changed her mind so many times and wanted everyone to be there - but the only fair way is to stick to the original list.
Just apologise, apologise, apologise.
State it's a learning curve for both you and your daughter.
If you do it right, you'll get a lot of support and sympathy from the other parents.
But that's assuming some of the invites are in childish hand-writing - and your daughter did this.
If something has happened and you can no longer afford a party - just say grandparent illness/ family issue and cancel. And say you'll make it up to everyone next year. And move on.
But PS: Never, ever accept responsibility for a child's sibling. A;ways, always state that it's not a problem, but the parent obviously needs to stay, pay and be responsible for the other sibling. That is always non-negotiable.