Is it lawful for a woman's guardian to marry her to his son?
Q: My wife died after giving birth to two sons.
After my paternal cousin's death I married his widow who had bore him a daughter. I am now the guardian of the daughter. Is it permissible for my son to marry this daughter of his late uncle
knowing that they are not related to each other through foster relations?
If the reality is as you mentioned, it is permissible for your son to marry that daughter. The validity of the marriage is not affected by your guardianship of the girl. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.