He wants to kill his wife’s relatives after they robbed him and took his wife by deceit
Question

                A man married me to his sister in front of two witnesses, one of whom was his brother and the other drew up the marriage contract. I saw the woman and she saw me before the marriage was performed, and her mother and all her family knew that I was of a different nationality than them. The woman, her family, her guardian and the witnesses all accepted that, and the marriage contract was done. The man told us that the woman had gotten divorced from her previous husband whom she had not seen and he had not seen her. After the woman got pregnant from me, her brother started to demand money from us and we gave him enough money to live a good life with his family, but he asked for a lot of money and I refused to give it to him. After the woman got pregnant and two months before she gave birth, he came and asked my wife to go and visit her sick mother (and he took her away by means of this trick), then he gave her to another man and said that he was her original husband, and that my marriage to her was not valid; the man was of sound mind and mature and had knowledge of Islam. Before I take any action against him and his family because of protective jealousy concerning my honour, I want to ask about the Islamic ruling on this calamity that has befallen me. Please note that my wife has given birth and they said to me, this child is your daughter. How can the child be my daughter if my marriage to her was not legitimate, as they claim? Please advise us, may Allaah reward you, before I kill this family, old and young, and in sha Allaah I will follow the Islamic ruling and the fatwa of the scholars of Islam.
Praise be to Allah.If her brother was her 
    guardian, then the marriage is valid and they have no right to take your 
    wife away from you. This is wrongdoing on their part. They had no right to 
    harass you in order to take money from you. The wealth of a Muslim is not 
    permissible to anyone unless he gives it willingly. 
You have to go to the 
    sharee’ah courts and turn to the scholars and prominent figures in order to 
    resolve this problem. 
With regard to killing 
    him and killing any member of his family, all of that, as you mention in 
    your question, is a major sin that is worse than blowing up the Ka’bah. 
Whoever does that, the 
    curse of Allaah, the angels and all the people will be upon him, and his 
    recompense will be Hell, to abide therein for eternity. 
We ask Allaah to keep us 
    safe and sound, and to set the affairs of the Muslims straight.