Firstly, you have to find out whether she is the first wife of your father or your mother is his first wife. Your mother won’t have any legal rights as if she is his first wife. If your mother is his first wife then you have the right to take legal action against your father under Hindu Marriage Act.

The Hindu Marriage Act, 1955, prohibits polygamy. Section 5 of the Hindu Marriage Act, 1955, specifies that at the time of marriage, none of the parties should have a living spouse. Therefore, if this condition is not met, the second wife does not have the right to claim any share in the property of the husband. Additionally:

If the second marriage is valid, i.e., the husband gets married after the demise of the first wife or after getting divorced from the first wife, then the second wife has the same rights as the first wife over the husband’s property. This is valid for both the husband’s self-acquired as well as ancestral property.
Laws related to the property rights of the first and second wives and their progeny are highly nuanced and require the judge to invoke multiple Government Acts to validate a justifiable verdict.

So, unfortunately, you only can confront your father about this at first and then can proceed with other things.