Marrying one's father's daughter

Q: My mum got married to my dad when she was very young. I was a result of that marriage that didn't last. After few years my she married a man that became my dad in every way. I got married and had kids my wife is a cancer patient and gave me permission to mary again. On the night of my walimah I found out that my dad got married again after my mum and had 2 kids a boy and a girl. He got divorced again and married another woman from whom he had a daughter who happens to be my second wife. We already consummated our marriage. I am so confused.