The costumes were wrong for the era, plus...

So were the accents. Kerr's character from England made me want to laugh each and every time he opened his mouth. Same goes for the doctor. No attempt at all was made to present these characters as a genuine product of their time, save for Price of course. Don't get me wrong, as I think he did a fine job for the most part, but the brother and the doctor were so modern-day America that it did make me laugh a bit. This is a much more prominent problem than the costumes, in my opinion. Oh well, the film is excellent aside from these small nitpicks.

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Whose idea was it for the word "Lisp" to have an "S" in it?

He probably wanted to take advantage of the fact he was filming in color and using red touches would add to the ghoulish proceedings; probably also to keep expenses down by reusing props from his previous films.

I watched "The Fall of the House of Usher" from 1960 and this movie back to back and much of the sets and props were re-used. See also the red glass wine goblets on the table.

I told you a million times not to talk to me when I'm doing my lashes!

Red does seem to be the best color for Gothic horror. Most Gothic horror films I've seen have lots of red. As for the costumes I don't know a lot about history anyways so it doesn't bother me any.