For The Sims 3 and Sims 4:

Your Gonna have 4 sims characters.
Two are gonna be husband and wife(you can make them Young Adults or Adults)
They are gonna have A toddler that pretty much all you have to do is take care of it.(It can be a boy or girl).
The 4th sim cannot live with you it has to be a woman.
(Sims 3)
The woman does not have to be in another household you can simply can go back and forth across town.
This woman has to be a maid. (Young Adult/Adult)
(Sims 4)
You can just simply make believe that she lives in another house. She would have to live near you.

The jobs except The Maid and Toddler can be whatever you want. Making the house make sure to add a guess room
Objective:
For the first three days and function like a normal family! On the second day call your maid over to clean. Leave your house dirty (Don't make beds, dont clean up plates,etc.) Sixth day the husband needs to take work off and the toddler should be sleeping if the wife does not have work that day make her go out and have some fun. As the maid is cleaning try to make the husband flirt with her. At first try to make the maid not like it (this does not have to be at home it can be at a bar and you can send her a drink or something.)
On the eight day they become. Closer( I mean really close.)
Nineth night make them woohoo in the guest room.Make sure the wife does notice.

When your ready this night make them try for a baby and this time make the wife get up to fufill her needs if she has any. She hears noises in the guest room and goes in there but does not get caught she peeks in there (If you do repeat over and over until you do)
Later on the next day pretend that you are talking with your husband about what you did last night. You don't get mad you just say that him to break up. The husband obeys because you fill wrong doing woohoo with someboby else

The next day the maid plans to do it with you again but you turn it down. She flirts with you more than she cleans. She's obssessed with you.
Some point of the challenge. The husband tells the wofe about it and then your husband goes to work. The wife does not.
Make the wife and the Maid have a fight that last until the husband gets home from work.

As the maid is preganant she confesses to both of them who is shock and says to the wife that if you kill her she will never forgive her self. (Your choice if you want to if you do then stab her with Dexter the bear. ) if the wofe decodes to accept this make the maid kill her but make it seen like an accident that so the husband does not get mad at you.
Then have the after you had the baby male the toddler who does not like her new mother that the husband marries kill the baby. That will be the end of your challenge.

I would hope to see this on youtube or pictures I was going to name this My Husband's Affair but Obssessed is better because the Maid contiounsly ask the husband to do stuff with her. Comment below!!!!