Considering that the attack on the homestead was meant to look like being the work of the Tusken, who neither own nor use flamethrowers and especially not disintegrators, and we don’t see both kind of weapons in the Sandtroopers arsenal for this mission, the most likely weapon that caused the condition of Owen and Beru’s corpses are blasters.
Blasters were shown to be able to turn people into charred corpses a short time later in ANH, when we see Han shoot Greedo in the Mos Eisley cantina. Perhaps another reason for this scene, beside giving him a reason to travel with Obi Wan and Luke to Alderaan.
And why would Sandtroopers, trying to imitate a Tusken raid, give their victims a funeral? We never seen or heard about them doing this anywhere.
And simply burning along with the homestead also doesn’t make sense, because the corpses lay outside and not in one of the destroyed rooms of the moisture farm.
And people often forget that George hadn’t the idea of stormtroopes using thermal detonators, flamethrowers or disintegrators when he made ANH or at least didn’t introduced them at this point. The only hand weapons we see anyone use in the movie are blasters, gaffi sticks and lightsabers.
So i think highly powered blaster fire from more than one Stormtrooper was the most likely cause for Owen and Beru Lars becoming charred skeletons.
At least that was most likely the reason George Lucas had in mind, when he created the first movie.