Maybe after his step brother died (who is blond) her mother somehow made him change his hair color.
It makes sense, so practically the mother wants the ML to act as her real son because in reality she didnt accept that both of her real son died.
Additional theory: What if ML is also their real son? What if the other twin who supposedly died didn't really die but was abducted or whatever shits happened back then? Or ML's real parent is a relative of the family who adopted him?
Maybe after his step brother died (who is blond) her mother somehow made him change his hair color.
It makes sense, so practically the mother wants the ML to act as her real son because in reality she didnt accept that both of her real son died.
Additional theory:
What if ML is also their real son? What if the other twin who supposedly died didn't really die but was abducted or whatever shits happened back then? Or ML's real parent is a relative of the family who adopted him?