Father transferred ancestral property share to step bro. How to claim?

You can challenge the transfer as its your ancestral property and your father can only transfer his portion of sale. The  pre-requisite condition required for a property to be Ancestral property is that it should be owned by the great grand father followed by the grand father, father & present generation all in this linear arrangement, living & enjoying the property.

The property should not have ever partitioned & share in it distributed to any of the Hindu Coparceners as in any linear generation as mentioned above, in other words, it should be intact single property which is being used jointly by all members of the Hindu Joint family.

Therefore, all property inherited by a male Hindu from his father, father's father, or father's father's father, is an ancestral property. The feature of inheritance of ancestral property according to the Mitakshara law under Hindu Succession Act is that man inherits from any of his three immediate paternal ancestors, namely, his father, father's father and father's father's father is ancestral property as regards his male issue, and his son acquires jointly with him an interest in it by birth. Their rights attach to it at the moment of their birth.

Therefore, you have a share a file a suit for partition thereby seeking share in the property, also seeking cancellation of document. In addition to this file an application for temporary injunction thereby seeking stay on the property which implies no one can sell or make modification in property