Hello,Yes, if the house is jointly owned, with your step mother it automatically becomes hers upon your father's demise.This is known as the survisorship rule.
You need to check that it is indeed a joint tenancy and not a tenancy in common. Can I assist further?
All we know is that it is in joint names - how do we check the tenancy? The govt website also says that is there is no will even if the property is in joint names my stepmother only inherits the amount up to the inheritance threshold and anything above is shared between her and children? You have not answered this or anything about the question I posed ie do we need to go to probate?