I have the impression that formal terms of address were more often used, even in private, than we would find appropriate, though obviously this would vary greatly. I think this tendency towards formality is not so surprising in a household such as MP, where most of them seem to be obsessed by the nuances of social status.

I assume your comment refers to the conversation in Ch.3 when Lady B and Mrs N discuss the possibility of Fanny going to live at the White House. I think there may be more going on here than just habits of formality.

When Lady B raises the subject she addresses Mrs N as "sister", which is pretty neutral.

Mrs Norris is absolutely determined to avoid the trouble and expense of Fanny. In her voluble arguments against the idea she uses the following terms of address:

"My dear Lady Bertram"
"Dear sister"
"Lady Bertram"
"Lady Bertram"
"Lady Bertram"

I think Mrs Norris is using these terms as a verbal weapon here. I always hear her say "Dear" and "My dear" through gritted teeth, using the endearments as a way of softening up her opponent.

When she repeatedly addresses her sister as "Lady Bertram" I think her sub-text is something along the lines of: "I will concede that your social status is higher than mine, but there is absolutely no way that I am going to agree to what you suggest. Forget it!"