It is a little doubtful, but on p.1, frame 5, there is the thief and he looks quite alike Balthazar's brother on p.58, frame 1+2+4. So I assume it was Balthazar himself who stole the fetish, took it home, hided the original one in a trunk and had the second copy stolen by his murderer Tortilla.
He does look suspicious, I'll grant you that.
However I don't think he looks that much like Balthazar's brother; on top of that he appears to be wearing a bowler hat whereas the thief hiding in museum is not.
Golf Tango Fox:
You have just reminded me of something that always troubled me with that story. The fact that Tintin actually allowed Tortilla to be murdered.
This bothers me a fair bit too. Surely it would have been enough to catch them in the act of breaking into the cabin? At first I thought that it was down to Herge's episodic plotting from one page to the next still happening, where he didn't even know what was going to happen next - but the murder happens at the top of the page. Tintin doesn't overhear them plotting to kill Tortilla, just that they know where his cabin is.. so it's possible that he just wasn't expecting him to do that.
BTW - blackface disguise - I remember reading how for American audiences black people were re-coloured in some editions. Guess they wouldn't have been able to for this one, but maybe the fact that it is so sambo-like made that okay!