Does a girl marrying a man who is already married to a foreign woman and has two children with her, constitute an injustice to his wife and their children, and what is the Islamic legal ruling on the husband in this case?

There is no objection to a Muslim man marrying a woman from the People of the Book, provided he can be just. He must invite his non-Muslim wife to Islam and encourage her to embrace it, lest her disbelief influence their children. If he divorces her, she has no right to custody of the children due to her disbelief.