By virtue of his father being shot and basically incapacitated, Sonny was 'acting' Don and therefore in charge. Everybody knew that. That may not be how it works in real life...but it's the way it worked in The Godfather. It was why Sollozo had Vito shot...so he could do business with Santino. Of course he was the Don. And he was a bad one, as his own father admits years later.

And I don't know about anybody else...but when Sonny died I saw plenty of emotion coming out of both Vito and Tom (in one of the best scenes in the film, by the way). Vito says that his wife is upstairs crying. In Italy, when Michael is told of Sonny's death he remains calm but you can certainly see the emotional impact to both him and Don Tommasino. What more do you want? The reason we see 'no emotion' from the others is that there are no scenes depicting Sonny's wake/funeral and the immediate aftermath of his death. This does not mean we can take for granted there was no emotion from the family.

So no...can't say that I 'missed' that Sonny was technically 'The Don'.

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