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. Tristan's been looking after Amari ever since ... and now he's officially been appointed his brother's legal guardian.
As we reported ... in his original petition, Tristan claimed their father, Trevor, has been absent from Amari's life ... leaving Tristan as the closest family member available to step up and provide the care Amari needs.
Tristan's got a lot of responsibility here ... Amari is unable to care for himself because he's been diagnosed with epilepsy.
Amari's also getting $103,475 in inheritance from his mother's estate ... and Tristan said in his earlier petition that he would protect the money from loss and provide for Amari's care.
Good on Tristan.