Since there does not seems to be any stories regarding this cheating plot.
Plot: This is in the victorian era. Bingley and Jane are married, so is Darcy and Elizabeth. Darcy is a workaholic owner and is always out with work and does not spend time with Elizabeth. When Jane and Bingley visit pemberly, she tells the same to Jane and she discusses it with Bingley so that Bingley can speak with Darcy. But Bingley noticing that because of Darcy’s absence, Elizabeth is lonely and sex deprived instead starts to seduce Elizabeth and finally have sex with her.
Requirement:
Elizabeth Should be Busty (as in 1995 Series) and not like the movie
While Elizabeth is being Seduced, Jane realizes what her husband is doing and instead of disapproving encourages Elizabeth to take advantage and have sex with her husband. She does this because of two reasons, she loves both Bingley and Elizabeth and also wants to get back at Darcy due to his interference at the start of Bingley’s and Jane Acquaintance
Darcy does not realize what is happening
Optional:
One of the scenes can be when Elizabeth and Bingley during one of Elizabeth walk in Pemberly’s garden, it rains and both get wet and take refuge in a nearby empty cottage (like in the movie). Because of rain, Elizabeth dress becomes nearly see- through and Bingley is unable to turn away or just keep looking at it and failing to hide it. Elizabeth notices it and due to her nature starts to tease him verbally, even though she is embarrassed. This Banter leads Bingley to ask Elizabeth to show her breast and from it leads to sex
There can be a wager involved between Elizabeth and Bingley for which Bingley wins and ask Elizabeth for some sexual favor (Blowjob, Handjob or See her breasts)
Jane Complaining to Elizabeth about Bingley’s sexual appetite to Elizabeth causing her to become Hot and bothered.
Jane catches them during sex and joins them to a threesome scene
Outdoor sex, since there will be no-one in Pemberly Woods