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.

Marrying a shia

Q: Is the Nikaah between a Sunni girl and a Shia boy halaal or not? The mother of boy is Sunni and the father is Shia. The same with their grand parents. They lived happily without interfering with each others maslak. They ask to do the Sunni nikaah and every thing according to our maslak. Please guide me with proper reference. The family of boy is not staunch. They don't do matam but they do disrespect the Sahaba ikram.