What is the ruling of Islamic law concerning a marriage that took place with the consent of the parties, witnesses, and guardian, but then the brother claimed its invalidity after the wife became pregnant, knowing that the child was attributed to the first husband?

If her brother is her guardian (wali), then the marriage contract is valid. The wife's family has no right to take her away or extort money from you. This is oppression and aggression, and you must resort to the Sharia (definition) courts, scholars, and respected figures to resolve the issue. Killing any member of her family is a great injustice, and the recompense for its perpetrator is Hell, wherein he will abide eternally.