My husbands extramarital affair with cousin sister

I am a Hindu and married to a Hindu guy since 9 years back. Before marriage my husband had relationship with his fathers brothers daughter and has continued after marriage also. I got to know about this 2 months back, He says after our marriage he has secretly married her at home when no one was there. I am not sure about this he would have married before marrying me also I feel he is not telling truth in this. They had physical relation before our marriage and continued till now. His cousin sister tells that her marriage is valid and she is her first wife. According to Hindu marriage act marrying a fathers brother daughter or cousin sister is valid ? Also tell me how my husband is punishable in this. He is a central govt employee.