If there's one thing Western society has agreed on for at least three generations (IMO since Freud's ideas became popular), it is that parents should never admit to having a favourite among their children, and should do their very best not to *show* any favouritism either, for fear of the fatal effects this might have on the other children's self-esteem. However, I suspect that in JA's time, both Mr. Bennet's obvious favouritism towards Elizabeth, and Mrs. Bennet's towards Lydia, would not have been considered as "dysfunctional" as we would see it now. It might have been considered not a good idea to be too blatant about it, but accepted as a natural thing that a parent would prefer one child over another. Also, I know that in general it used to be more common for parents to be tactlessly honest about their children, partly because instilling humility was considered a good thing.