1) if your father bequeaths flat in your name your sisters can dispute will on ground that will was executed under undue influence or that your parents were of not sound mind at time of execution of will
2) get will drafted by lawyer . have it registered and attested by 2 witnesses . if you take legal precautions chances of your sister succeeding are bleak
3)if your parents bequeath flat to your sisters you can challenge the will but if adequate precautions have been taken in drafting and execution of will your chances are bleak