MB made an interesting point in another topic, the one titled "i'm doing 1840...finally". It's one of the more recent posts in the topic, his latest one anyone.
Basically, they're talking about how Edith is killed in 1840, which breaks the continuity of Edith being very old and alive in 1897, but because the writers were writing the series from the POV of Barnabas and his timeline--that Barnabas traveled to 1840 after 1897, and therefore it came AFTER 1897 in Barn's personal timeline--that this is perhaps why they made this mistake.
This made me think of an interesting point in that, since in the DS world, Barnabas traveled to 1897 first, where Edith was alive, then traveled to 1840, where Edith was younger and alive, then was killed, perhaps because of Barnabas traveling back to 1840 in the first place. It's all very confusing, I know, but the basic point of my idea is that in 1897 Barnabas scared Edith to death before she could tell anyone the secret, then when he went back to 1840, possibly causing Edith to be killed just because of his preseance, that this is why the secret was never told. Not only did Barnabas scare her in 1897, but he inadvertantly caused her death in 1840, thus preventing a future Edith from telling the secret in 1897.
Of course, this really relies on possible parallel times--for instance: Barnabas starts in the future of the parallel universe that has the 1840 events in it, and since Edith was killed when she was young in this time, the secret never got told to anyone. But when he traveled from this present day time to 1897, he went into the 1897 of the regular time band, where Edith never died at the hands of Gabriel, and Barnabas instead had to scare her to death and prevent her from telling his secret, in case he returned to the present of the regular time band.
Also, this would require the two time bands to be very closely related and running together concurrently.
Hopefully this came across more clearly than it seemed in my head. This crap really gets convoluted when you get into the stink of it.