Can anyone recall if Sybil's grave stone has actually appeared in an episode yet?
The grave stone I'm talking about is the prop that we can see in this paparazzi picture: here.
Personally I can't recall seeing it in an episode - or at least not close enough to see the detail of the wording that this prop shows. I don't think we saw it when we saw Matthew's grave stone being erected.
Now this is a very important point, as this prop is what calling Robert Crawley the 5th earl of Grantham is based upon. If I recall correctly, other evidence points to him being the 7th Earl - this grave stone prop was what pushed the number back to 5th Earl.
If the grave doesn't appear in an episode with the detail "Daughter of the 5th Earl of Grantham" clear - then it isn't canon information.
The reason I say this is because although this is a picture of a prop for Sybil's grave, it isn't an official picture of it. It's a paparazzi picture of it - an unofficial picture taken without permission.
This means that just because the stone shows the "5th Earl" we can't take it as canon unless it appears in the show. Because we have no way of knowing if the show makers ever intended this prop to be seen in such detail.
So, what is everyone else's opinion on this?
I haven't read it yet, but if anyone has access to the latest behind the scenes book, it would be interesting to know if there is any information on this issue in there.
ETA: The image in question has been deleted from this wiki as it is a violation of the photographers copyright. Please do not reupload it. However, you can view it here: 'Downton Abbey' Series 4 Pictures - Maggie Smith As The Dowager In Graveyard Where Lady Sybil And Matthew Crawley Are Buried