Miguel insists he's Ernesto de la Cruz's relative
But what gets me is, his whole family (except, of course, Hector) would know that he wasn't. So why didn't they tell him the families were totally unrelated? He knows he's a Rivera, so why would he think he's a (de la) Cruz? Of course, when Miguel sees the whole photo in the ofrenda, he connects the guitar in it with the one he sees Ernesto with, so he naturally puts two and two together with the logic of a child. But he doesn't realise that it is NOT Ernesto with the guitar in the photo.
My question is: Did the Rivera family do that good of a job in ERASING Hector from their history? It would seem so, but they forgot the folded bit of the photo with the guitar, so maybe not. It just caused them more initial problems, as we've seen.