My mother owns a property (self acquired) & wants to pass on the property to me (I am her son) after her death. However before that she has written in the registered will that my father will be owner of the property (after her death) & after my fathers death the same will needs to be executed (that she wanted to give the property to me).. Can this registered will be challenged by my elder brother?

Or if she gives the property to me directly via registered will, then it cannot be challenged by my elder brother?

2 different questions/options actually..