Those women with whom nikaah is not permissible

Q: I want to get married to a Christian girl and she is even willing to convert to Islam now. She is not Muslim yet, actually she was with a Muslim boy before and he took her to the mosque to do nikaah as she only did it because that boy wanted her to go to mosque with her and they did as well but as per her she did it because that boy wanted to do nikaah with her according to Islam. The permission of her mother was also not in it.

My question is, was the nikaah valid or not, as she said yes in the mosque in front of the Imaam. If the nikaah is valid then what should I do?

Q: I know a person who is married to 2 wives. The relation between the two wives is aunt and niece i.e. the second wife is the first wife's niece (that means his first wife's brothers daughter). The second wife had physical intimacy with the man before his second marriage. Does hurmat musaharah takes place in this case? Is the second marriage valid or not?

Q: Can a husband marry his stepdaughter or even his own daughter and the marriage still be valid with his wife?

Q: If a person commits zina with a woman and later on do nikah with her daughter, what must he do now?

Q: Is it possible to marry an ex-wife's daughter? I know a man that married a woman 13 years older and she never told him she has a daughter almost his age. They are not together any longer and he wants to propose to the daughter. Is this permissible considering he didn't know this? He didn't have intercourse with her since finding out as he was very hurt and shocked.

Q: I've been working in corporate company for a few years, and have met a girl there. We both really like each other. She however, isn't Muslim. She's a Christian. She really has a great relationship with God. But, she doesn't have intention of converting - even though she's considered it a number of times.

Her ideal way of raising our children up would be to teach them both religions, and focus on their similarities. I will also always put Islam first, but, if this is allowed, I really don't want to let go of this girl. We really get along well, she's intellectual, she's a few years younger than I am, and we both love talking about religion and life in general. Similar interests. Please advise me as to what to do.

Q: Is it permissible to marry one's mother's aunt in Islam?

Q: Can I marry my sister-in-law (wife's sister) as my second wife while I am still married to my wife?

Q: Can a Muslim man marry a Christian woman? Does the girl have to convert to Islam?

Q: I am from India, non-Islamic country. I love a Christian woman. She is chaste and religiously righteous (prays daily, neither eat pork nor do alcohol, etc.). But she considers Jesus as god. Am I allowed to marry her, even though, what she does is considered shirk?