Why does Luke Skywalker seem as upset by Obi-Wan's death, as he is by the death of his uncle and aunt?
Especially considering he found them burnt to the bone (the people that raised him all his life to that point) as opposed to Kenobi (up until recently regarded as "the hermit that lives across the Dune Sea") vanishing and becoming one with the Force (possibly not completely dying).
We don't know which deaths affected Luke more.
However, as to your suggestion that Luke seems to be more upset about Obi Wan's death, I think there are a few possible explanations:
Luke saw the mission to save Leia as a way to avenge Owen and Beru. Being able to do something makes the grief more bearable: "Those imperial scum killed my family - let's get 'em!" Contrast this with "That Vader bastard killed Obi Wan - RUN AWAY!"
In a similar way, rescuing Leia and learning about the Force on the way gave Luke something to keep himself occupied. He had less time to dwell on the past. After it was over, he was finally able to absorb and process everything that had happened over the past week or two. This is when the deaths of Owen and Beru finally sunk in. His bereavement at this point is about Obi Wan and Beru and Owen.
When Owen and Beru were killed, Luke still had a tie to his former life - Obi Wan, who knew him, his dad, and his aunt and uncle, and who had been (unbeknownst to Luke) watching over him his whole life. After Obi Wan died, Luke was totally alone, with no ties to his former life, which must have been crushing. Luke's link to Tatooine was totally severed. He was on his own, and he had no idea what to do next. The future was a complete unknown.
We don't see every moment of every day, and much of Luke's grieving may have taken place off camera. The movie would be less enjoyable if it consisted of scene after scene of Luke moping. He's already whiny enough, any more griping and moaning - however understandable it might be - would run the risk of making Luke an unlikable protagonist.
The script describes Luke's reaction to the Lars' deaths as:
The speeder roars up to the burning homestead. Luke jumps out and runs to the smoking holes that were once his home. Debris is scattered everywhere and it looks as if a great battle has taken place.
Luke: Uncle Owen! Aunt Beru! Uncle Owen!
Luke stumbles around in a daze looking for his aunt and uncle. Suddenly he comes upon their smoldering remains. He is stunned, and cannot speak. Hate replaces fear and a new resolve comes over him.
And it describes Luke's reaction to Obi-Wan's death as:
Vader brings his sword down, cutting old Ben in half. Ben's cloak falls to the floor in two parts, but Ben is not in it. Vader is puzzled at Ben's disappearance and pokes at the empty cloak. As the guards are distracted, the adventurers and the robots reach the starship. Luke sees Ben cut in two and starts for him. Aghast, he yells out.
Luke: No!
The stormtroopers turn toward Luke and begin firing at him...
...
Luke, saddened by the loss of Obi-Wan Kenobi, stares off blankly as the robots look on...Luke looks downward sadly, shaking his head back and forth, as the princess smiles comfortingly at him.
Luke: I can't believe he's gone.
Regarding his aunt and uncle's deaths, Luke is stunned, speechless, hateful, and resolved. For Obi-Wan's death, he is aghast, saddened, and in a state of disbelief. His reactions are different, but they seem approximately equal.
Given that Luke was closer to the Lars, one would expect him to be clearly affected by the Lars' deaths more than Obi-Wan's. However, there are at least two good reasons why Luke would be more affected by Obi-Wan's death that one would expect.
When Obi-Wan and Luke found the dead Jawas who had sold the droids to his uncle, Luke realized that his aunt and uncle were probably killed by the stormtroopers who found the Jawas:
Luke: If they traced the robots here, they may have learned who they sold them to. And that would lead them home!
Luke reaches a sudden horrible realization, then races for the speeder and jumps it.
Luke had some time to mentally prepare for what he would find at the Lars' moisture farm on the ride to the farm. Furthermore, while the scene at the farm was gruesome -- only the skeletons of his aunt and uncle remained -- it was in a way easier for him that the bodies were not identifiable.
On the other hand, Luke personally witnessed Darth Vader cut Obi-Wan down. When he saw this he screamed "No!", but this scream was probably just as much due to the shock of the sight as grief over the loss of Obi-Wan (if not more). Thus, Luke's seemingly strong reaction to Obi-Wan's death was partially due to his surprise and having personally witnessed it.
By the time of Obi-Wan's death, Luke had had time to learn from Obi-Wan and see some of a Jedi's powers (using a lightsaber, the Jedi mind trick, etc.). This gave him hope that the Empire could be defeated -- he didn't like the Empire to begin with, but he really hated the Empire after what the stormtroopers did to his aunt and uncle. Furthermore, Luke saw Leia's message to Obi-Wan in which she said that Obi-Wan was her "only hope". Luke had a lot of hope pinned on Obi-Wan that the Empire could be defeated, but instead he saw that hope dashed when Obi-Wan was killed. Darth Vader cutting down Obi-Wan represented not just the death of an old hermit, but the seemingly invincible Empire's triumph over one of the last remaining Jedi and what Luke thought was their "only hope" to defeat the Empire (Luke did not yet know that he was the "new hope").
