Hello! I have adapted the rules for The Sims 4 Big Sister Challenge, so they could fit for The Sims 3! I give full credit to poballa on mod the sims, the original creator of The Sims 4 Big Sister Challenge. (The link: http://modthesims.info/t/548184 )
The Rules:
•The teenager must be a female, but the child can be whatever gender you'd like.
•you must start out with a 20x15 bulldozed (empty) lot.
•The siblings must not interact with anyone else but teenagers and children, no adults or elders, until the older sister grows into an young adult.
•The older sister is not allowed to go to school, but the little sibling can.
•The older sister can have a job, but it's not required, you can make money through hobbies, ie. painting, writing, gardening, etc
•When you start out in the empty lot, you are allowed to spend 6,000 simoleons building and furnishing the house.
•The house must contain the following: 2 bedrooms (one per sibling), 1 kitchen, 1 study (for hobbies and homework), 1 bathroom. If you have remaining money you may spend it on a living area only.
•You may spend 1,000 simoleons building and furnishing a garden, containing at least 2 potter boxes. And leftover simoleons can be used to add more to the garden.
•The lot must be in Apaloosa Plains
•The Sim Lifespan must be long.
•Must kill the mother or father sim.
•The mother or father is allowed to come back as ghosts and interact with their children.
•To travel, you must first purchase a flamingo and keep it in your family inventory.
•The teenager can create relationships, such as lovers and enemies if you choose to do so, but they are not allowed to have children until the teenager turns a young adult.
•You are allowed to move houses if you please, and have the funds.
•The younger sibling may not move out at all, unless he or she dies.

The Bigger Sister:
•Must be a female teenager
•Must have the Surrounded By Family Lifetime Wish
•Can have any traits except Hates children, Evil, and Mean Spirited

The Little Sibling:
•Can be any gender child
•Can have any traits

The Parent(s):
•You can create only the mother or the father, or both if you desire.
•The parent(s) can have any Lifetime Wishes and traits.

Have fun with this challenge. If you decide to create a video and upload it, please tag it as TS3 Big Sister Challenge. If you want to use this idea for any other version of the sims, do as you please, just leave credit. (To the original creator. I mentioned her at the beginning of this)Don't forget to create a storyline or plot for why the two siblings are how they are, how their parents died, etc. Remember the rules, and enjoy! If you have any questions comment below.