Huh. Didn't consider that possibility. I feel like Hitchcock would have made it explicit if that were what he meant. No reason not to. But your interpretation is plausible and compelling irrespective of whether he intended it or not.
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Also he had in his possession the framed photgraphs of his and Emma's parents. Which Emma immediately recognized, even down to the frame.

With regard to #4, one person alluded to the fact, but nobody directly mentioned, that it was Ann who said he looked "different". Joseph's comment was that he looked "great". That Ann remembers him at all would seem to indicate that the family had seen him within the past several years, so it hardly seems likely that anyone would be able to pull off a switch. It might be different if the nobody had seen Charles since he was a boy.
As for some of the others:

1. Though I'm sure it wasn't already the case in'43, by now it's the stuff of myth that everyone's first response upon finding out their neighbor is a serial killer is to say how nice and normal they seemed. I didn't take this as anything more than a recognition that if serial killers didn't look like nice people, they wouldn't be likely to get away with their crimes.
2. If Charles knew they weren't related, but Charlotte thought they were, any sexual tension would still be half as weird, which is plenty weird enough.

Young Charlie knew her Uncle Charlie, and so did her father. Emmy had seen him as an adult. The purpose of the bike story is to show that a brain injury changed him from an outgoing child to a quiet brooding boy, who grew up with no conscience.
He was the real Uncle Charlie.
I guess it's like looking at clouds. You see one thing and I see another. Peace.

The 1991 Mark Harmon made-for-tv remake did have him turn out to be an imposter. It robbed the story of a good bit of its effectiveness, but I suspect that the writers were trying to hint at the incest subtext a bit more openly (Cf. the scene where Charles kisses Young Charlie good night in her bedroom: on the lips, and just a bit too lingeringly); but because of television censorship, they couldn't have it turn out to be actual incest. So 'Uncle' Charlie turns out to be no relation to the rest of the family at all; just a clever (and apparently very well-informed) impersonator.

Interesting thread, but I think the sister would have surely known if he was an imposter. Also, in the dinner scene where Uncle Charlie is opening the wine bottle, they do laugh about some childhood memories while the husband looks on with a sort of bored expression. The sister says something like "Remember when the jones girl got married".

I agree. I think AH suggested this in the scene where Charlie claimed never to have had a photo taken, which his sister and niece then rebut.

1. Family members say these things even about terrible family members to keep up appearances.
2. There was no sexual tension. They were near each other because of the camera and close-ups are more dramatic.
3. It's because he's a changed person. He's evil now. He doesn't think much about how he use to be.
4. It's because he's evil now. Evil people have a different attitude and facial expression.
   Unlikely, because it would have been revealed and would have be a better story line than the one in this film.

Exactly. Everyone is reading much too much into this. I agree with the "detachment" comment, that nothing he does nor any gift he gives means much to him. He has no genuine emotions or attachments, only survival instincts. And this was set in motion by his head injury as a child. In fact the picture we see of him is clearly Joseph Cotten.

so, the whole family woudln't recognize their own relative? they were obviously close knit, because she and the uncle had the "connection" and they had talked of him visiting before.
blah.
Swing away, Merrill.Merrill, swing away

I watched it years ago and thought they would reveal he was an imposter but it never happened.
now I watched it again today and nothing in the movie makes me think he is an imposter. weird. it's as if the movie changed somehow.
anyway, I don't think there was anything he could have found out about the family altho I always wondered if he knew the "real" Charles and got info from him.
That would have made a much better movie.

the other one who was killed by the police might be the real one. He could be innocent. This fellow might use his identity to come here and waited for his assassination. So that he did not show even simple affection to the family while he was threatened.

I noticed the thing with the dress to and wondered if that might not imply that Charlie was an imposter. I think it was pretty clear by the end of the movie that wasn't the case though. I think those scenes were meant to illustrate how detached he was. He didn't really care or think about the gifts he gave, he just enjoyed the thrill of spending the money of the widows he murdered and implicating his family by secretly making them beneficiaries of his crimes.

Actually I was sure he wasn't the real Charlie, and expected that to be revealed at some point, but it didn't happen.
He was using different names in the beginning and when he sent the telegram to Santa Rosa, "Uncle Charlie" felt like just another fake name. Then his relatives thought he was now different when they picked him up. Young Charlie's first phase is "Are you?". Little Ann says "I remember you sort of, but you look different". He doesn't remember Young Charlie's beautiful dress, which "he" had bought actually. There were lots of similar suggestions in that direction definitely, not to mention his constantly avoiding to talk about the past, not remembering the childhood picture, etc, but maybe the director just wanted to trick us.

I think you're reading way too far into it.

1. If there was any sinister significance of his childhood head injury, I'm sure that it was that it "tripped" something in his brain to make a latent darker aspect of his personality emerge; his sister Emma mentioned that before the accident, he read a lot, but after it, he stopped reading and became full of physical energy. I doubt Hitchcock was suggesting that somehow the family was given a different boy.
2. I think that tension is supposed to be off-kilter and unsettling.
3. I can't recall any instances of this, I'll have to rewatch. Maybe, if she talking about memories before his accident, it is just highlighting how his "original, good" self is lost to him.
4. Well, Uncle Charlie started murdering widows since they last saw him. Corruption shows.

That's an interesting idea. There was a picture of him when he was a boy that
looked like him. I would think his sister would know him. You might be right,
though. Next time I watch it I'll check your ideas out.
Marge