After 5 replies, you are not satisfied though the guidance is with more clarity.
If it is the self-acquired property of your father, he can deal with that property as he likes.
If certain disposal of ancestral property or some involvement of ancestral property, irrespective of its value is involved in the property to be sold now, the first wife son is having such rights for a share along with other co-sharers.
There is no clarity on the first wife and if the marriage for the second time is legal after divorcing the first wife.
If second marriage was during the lifetime of the first wife, without divorce, once again gifting of ancestral property to the second wife is not legally valid.
Contact a local advocate and defend if your eldest brother issues such notice.