Parents make a bet to see who can seduce their son/daughter first

[Elementalitis]

A mother and father have adult twins - a boy and a girl - who still live with them. The mother lusts after the son, and the father lusts after the daughter. Upon finding out that they both have these feelings, the parents make a bet to see who can seduce their offspring successfully first.

 

[John_Vandermeer]

Or the other way around: The fraternal twins seducing their parents.

 

[TanyaDeVayer]

What if their neighbors have twins as well and they get to know each other?

 

[1darknightowl]

I like the idea of the parents betting would be 1st twin to move on the 2nd.

I love the idea of twins.

 

[Lalrinkima619]

I feel like the roles should be reversed, brother and sister made a bet to see who can seduce mom or dad first (son-mom, dau-dad setting)... It would be much more challenging for offsprings to seduce the parents which will make for a more entertaining story

 

[GlennJenko]

Or the other way around: The fraternal twins seducing their parents.

It could be both. The twins set out to do the same as their parents, but with a twist: the sister has confided in her brother that she's gay and has a crush on their mother. The brother sets out to help his sister, and so ends up having to run interference on his father to leave his sister time to seduce their mother, all while the mother is trying to get to her son.