They mention Lydia needing to wear lace and it slipping, so stupid question, what's the lace for? Is it to cover her cleavage or her nipples or what? I know they didn't have bras at that time, and cleavage seems to be the right answer, but both Jane and Elizabeth show some cleavage, so what's up with the lace? Mary going on saying "Lydia's lace tucker slipped", huh, I didn't see anything?
I think the lace scenes are supposed to be funny but it's really not.
Lydia's lace doesn't slip in the novel, but women did use the practice of "tucking lace" during that time period.
See this link
http://www.phrases.org.uk/meanings/best-bib-and-tucker.html
In this case, Lydia uses a length of lace tucked along the inside edge of her bodice to play down the bulge of her bosom. (Her bodice fits tightly against the bosom, causing a slight bulge at the top edge of the bodice -- sort of like "muffin top" only it's the bosom instead of the waist. Tucking lace plays down this bulge, making it less noticeable.)
Sometimes, older female characters in Austen films are seen wearing a fichu
http://en.wikipedia.org/wiki/Fichu
Mrs Allen in NA07
http://www.janeausten.info/moviegowns/North2007/MrsA2007/mrsared.jpg
Mrs. bennet in P&P05
http://www.janeausten.info/moviegowns/PandP2005/MrsB2005/mrsbredlong.jpg
Mrs. Musgrove in persuasion 95
http://www.janeausten.info/moviegowns/P2/MrsMandLadyD/pmrsmmorecake.jpg
Another type of tucker is the sheer chiffon-type fabric tucked into the bodice by Elinor and Marianne Dashwood in S&S95
http://www.janeausten.info/moviegowns/sands1995/sands1995.htm
Dresses were cut low for evening, but a lady was expected to appear more modestly during the daytime.
I suspect that the reason why jane and Elizabeth don't need to tuck lace is that their clothing fits better so it isn't necessary. In one scene, the ladies are preparing dresses for a ball. Lydia will be wearing a hand-me-down gown that is being freshened up for her by adding new trims. But it doesn't fit quite right (Lydia probably has a larger bosom than her elder sisters), so her mother says she'll need to tuck some lace. Later, when Mary tells her Lydia's lace has slipped, Mrs B says "Lydia looks well" because she's allowing Lydia get away with making a show of herself. It illustrates how Mrs Bennet's lack of consistent governance over Lydia's behavior leads to her running wild.