If anyone knows the answer to your question it would be the CWGC. Otherwise it is not possible to generalise about what happened to individual soldiers. Every effort was made to recover the wounded, but often the only practical solution for the dead was a quick burial with the hope that it would later be possible to recover the bodies for a proper burial later. Soldiers were supposed to record the locations of burials, but in the desert it would be more difficult to record a precise grid reference due to the lack of geographical features. The Germans would also bury any dead they came across and then there was the problem of transmitting these details to the British. Sometimes this was done via the Red Cross, but in the heat of battle there was considerable room for error and for individuals to be missed.
One of the main reasons for burying the dead in temporary graves was to reduce the chance of the bodies being disturbed by wild animals. However a shallow grave did not offer that much protection against a determined animal like a desert fox, and so, gruesome though it sounds. it is quite possible there were no identifiable remains to be found later after the battle had moved on.
Not knowing the exact details of the attack involving your uncles, it's impossible to offer an explanation for what happened. Perhaps the vehicle they were in caught fire and so the dead brother was effectively cremated at the scene. If the surviving brother had to return to his unit on foot, he wouldn't have been able to carry his brother's body back with him, especially if he himself was wounded.
