Hi, welcome to the ehealth forum and I am glad to help you.
You seem concerned about the seizures in your step daughter.
You want to know if your daughter can have epilepsy after one seizure and no correlation on an EEG.
Epilepsy by definition is characterized by recurrent unprovoked seizures so single seizure is not sufficient to label the patient as epileptic. Further workup is necessary to reach a definitive cause for the seizures and you must continue follow up with your neurologist for the same. The other possibility in females to rule out is the hysterical attacks as you have described in your step daughter who threw a fit in doctor's office as she wanted to be told by the doctor that she had epilepsy.24 hour ambulatory EEG and MRI with epilepsy protocol can help the diagnostic workup.
Hope this helps. Take care.
Note: This post is not to emphasise final diagnosis as the same cannot be made online and is aimed just to provide medical information and no treatment suggested above be taken without face to face consultation with health care professional.