If Mrs. Long's nieces were part of her household, even for a visit, I would think they would be invited to every dinner that Mrs. Bennet invited Mrs. Long to, but Ch. 53 has Mrs. Bennet say:

[...] We must have Mrs. Long and the Gouldings soon [...]

which I am inclined to think will mean her alone. The young lady who says that to Elizabeth could be any number of people who think Elizabeth dislikes Mr. Darcy (most of her circle at Meryton?). They could even be thinking they are aiding her by making it awkward for 'that tall, proud man' to stay near the table! And is not a new Lucas lady due to come out, now that Charlotte is settled? Maybe this is Maria's next youngest sister. Or Maria herself...