1st Wife alive and my Father register married to a Widow,now her son asking property share to me

1. We are Hindu family, My mother is still alive and my father register married to a SC/ST widow which she already has a son from her first husband before, which we don't know this full story. 2. Later we came to know this news when my father spending more time in her house but forcefully we agree this and my mother not given complaint as my mother thought my father may get trouble legally and its prestige issue for our family. 3. Later for 10 years we kept quiet as my father left working and spending time with her in together 4. We fought and discussed many times but my father not at all ready to break this illegal relationship 5. Recently my father and his illegal wife fought for money matter and now he is saying they tortured lot, and illegal wife's first husband son starting calling me saying settle the amount and give the share of my grand-mother property. 6. Is this marriage still acceptable, my mother frustrated and planning to go court to dismiss this registered marriage. Please suggest, how we can deal this.