There is no cheat but it may happen automaticlly but the way i find easyest is to have a home with 8 sims one teen have them have a boy/girlfriend and then go off them and then it may happen. or sometimes that teen in your home will move to them so it is quite hard
There isn't a cheat but you can get mods that allow you to have more than eight people in a household, Awsomemod is one of the popular ones and I currently have it in my game.
If you do install it make sure you read the instructions because It can and will mess up your game If you do it wrong.
