No you can't, at least not without your step-grandfather's permission. What he has is called a "life estate". This is a possessory interest in a piece of property, such as a house, that lasts for the life of a person but ends upon their death. Generally a person who holds a life estate (the "life tenant"), has the right to do anything with the property that a full owner could do short of conveying any ownership interest in it. So, for example, a life tenant can rent out the property but cannot sell it. In any event, they have the right of possession and can dictate just who does or doesn't have the right to occupy the premises (at least during the life tenant's lifetime).Note: In addition to the life tenant there are also those that will own the remainder interest in the property (the "remaindermen") after the life tenant dies. If the remaindermen are concerned about the use of the property, they can try and negotiate to buy out the life estate from the life tenant.