I get the impression that Lucy *does* know what is considered good behaviour in polite society, by the way she is always trying to curb her sister. Yes, I think she (and possibly LM too) do feel vaguely inferiour to the Dashwoods, which is not a pleasant feeling for anyone, and adds to their dislike of them.

The line "They had too much sense to be desirable companions" reminds me of the one from P&P: "Lady Lucas was a very good kind of woman, not too clever to be a valuable neighbour to Mrs. Bennet."

I suspect that JA herself must have known more than one person who was intimidated by her intelligence!