1. The house appears to be self acquired property of your father,
2. if that is so then he can make bequest as regards the flat in any manner he chooses.
3. So in that event the Will creating Life interest in favour of mother and thereafter absolute interest in favour of you and your brother is valid and enforceable.
4. However the Will to make it enforce needs grant of Probate through a competent court of law. So in the probate proceeding your sisters may put opposition but once the Will is proved their objection goes.