Hi, As the will was executed by your father now your mother can't change the will.
2. As the property is the ancestral property of your father after demise of your father all the legal heirs have right over the property including two daughters and your mother.
3. So your mother and two daughters can file a suit for partition against your brother. Suppose in the suit if your brother come with defense that your father has execute the will then ask him to deposit the will in the Court so that you will able to get the copy of the "WILL". or otherwise you will get your share in the property.
4. You have one more options that your mother can gift her share to one of you, while executing gift deed.