My father nominated my name for society flat. can my sisters object ?

My father own a flat in western suburbs. I have two sisters and I am the only son of my parents. Recently my father made a will mentioning that he want to give this flat and my mother's jwellery (she passed away few years back) to me only as I am the only one who was been taking care of both my parents including their medical expenses and my both sister never contributed a single penny. Also inbetween my father had some clashes with both of them.My father put my name as nominee for the flat in society also.Just want to know can my sister's challenge this will, do I need to take any precautions