Final answer:
Bob granted his sister Mary a life estate. Mary can use the property during her lifetime, but upon her death, the property cannot be inherited by her heirs; it goes back to the original owner or a specified third party.
Step-by-step explanation:
If Bob grants his sister Mary an estate for as long as she lives, but her heirs cannot inherit the estate, the kind of estate being granted is called a life estate. A life estate is a type of property ownership where the individual, known as the life tenant, has possession of the property for the duration of their life. However, once the life tenant passes away, the property does not go to their heirs but reverts back to the original owner or goes to another person specified by the original owner, called the remainderman. The life tenant's rights to use the property are fully established during their lifetime, but they cannot determine the ultimate inheritance of the property after their death.