IS it even possible to hack whatsapp?
In any case remove the app from your phone > find another phone without whatsapp > install whatsapp and enter your own number (from the hacked account) > this should register your number again and kick out anyone else. After this you can download whatsapp on your own phone again and go through the steps
If it continues, I would suggest doing the same with her phone.
(Doesn't necessarily have to be a phone that never had whatsapp. Just find someone that's cool with you deleting their app so you can download it and use it to register your number)
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