I adopted a girl. When she became of age, I wanted to marry her to one of my sons. Is this Haram (prohibited) or Halal (lawful)?
It is impermissible in Islam to adopt a child; children must be related to their fathers. This is because Allah (Exalted be He) says:
Call them (adopted sons) by (the names of) their fathers: that is more just with Allâh. But if you know not their father’s (names, call them) your brothers in faith
As for your desire to marry your son to this girl, this is permissible if there is no impediment, such as breastfeeding that affects marriageability. You should also fulfill all the essential elements required in a marriage contract, which include: a Waliy (a legally accountable person acting for a woman regarding marriage), two just witnesses, and the bride's approval. Her Waliy in this case is the ruler of the country (Part No. 18; Page No. 160) or a person who acts on his behalf. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.