If your vet suspects a brain tumor, essential oils are probably not going to be of much help depending on how large the tumor is and what part of the brain it is located in. If it is malignant and she is already having grand mal cluster seizures, it's kind of doubtful that even medication will help much at this point. If it's a benign tumor, however, and it can be removed, then that would be the way to go and then determine which treatment would be best afterward.
It could be that it's simply idiopathic epilepsy. If that's the case, then phenobarbital has great success in controlling seizures, as does dilantin and a couple of other medications. Sometimes they are used alone, other times, in more severe cases they are used in combination with other meds.
Wait and see what your vet says after this course of treatment is done and then decide from there.
Many essential oils are extremely dangerous for dogs, often because they are too concentrated and/or cause toxic effects and symptoms. They should only ever be considered under the advice and supervision of a vet - and after researching very carefully the source and quality of the product itself. Oils produced for humans are not generally suitable or safe for use on dogs.