The reason for prohibiting women from visiting graves is the ahaadeeth of the Prophet (peace be upon him) mentioned in question #127. It is not because dead people can see their visitors. This reason is invalid and the rules of the world of the dead people are different from the rules dealing with the world of living people. If the reason for prohibition was the ability of the deceased to see people who are alive naked, then it would also be prohibited for men to visit the graves.
You need to encourage your mother and sister to make du'aa (supplication) for your father, as this offers the greatest benefit to the deceased, and it is much better than the mere visit. Tell them that this du'aa will benefit him whenever and wherever the person making the du'aa is, whether he or she is near the grave or far away from it.
We ask Allaah to have mercy upon your deceased, and to give you patiencefor your loss.
To Allaah we belong and to him is our return.