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 360aproko, the ruling came in Monday, about 5 months after Tristan first filed a petitionto 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.