1. if your brother is refusing to part with the Will then you can presume that she has left no Will at all.
2. if that is so then you are entitled to undivided equal share with your other siblings and your father, if he is still alive.
3. In that context you can file a suit for partition to claim your due share.
4. Once you file the suit for partition he then can be forced to disclose the Will to protect his interest.
5. You may note that making a false Will is a criminal offence.