Tristan Thompson is officially taking over as caregiver for his younger brother... because a court signed off on him becoming Amari Thompson's guardian.
According to new legal docs, obtained by TMZ, the ruling came in Monday, about 5 months after Tristan first filed a petition to become his little bro's guardian.
Remember ... Tristan and Amari lost their mother, Andrea, about a year ago when she died suddenly in Toronto.