Hello,
1) If the property was acquired as purchased property it is considered self acquired property and he can dispose out of as he chooses and no one can question it.
2) The brother's sons upon straining majority will by virtue of the sale agreement become owners of the property. As of now the guardian who would be your brother will be able to manage the property in his right as custodian of the property.
3) As such when your father is alive no one can stake any claim in the property and that includes you as well.
4) Your mother file a case of domestic violence against your father and seek maintenance, residence order and protection among other reliefs.
5) How was he able to marry your step mother while your mother was alive. If religion permits it is fine. Or else on a complaint from your mother he can be charged with bigamy.
6) Your mother can not stake claim of profit gained from sale of her husband's property.
7) As said earlier she can either file a Domestic violence case seek the reliefs or alternately she can file for divorce and seek maintenance. She can claim upto 1/3 of bus income.
8) You need to engage a lawyer who can take necessary legal steps to ensure all the rights your mother has from the marriage.