If i had brain tumor, but wasn't having seizures, would that be because xanax (alprazolam) is preventing the seizures?

Maybe: In general, xanax (alprazolam) is an anti-seizure medication. As such it would make you less likely to have a seizure, tumor or no, seizure disorder or no. This is true because there is such a thing as a 'seizure threshhold' and everyone has one. I cannot say anything about your actual state of affairs and you must rely on your docs. If you dont trust them talk to them until you do or get new ones. Be well.