I don't think it's possible to do without mods, but you can do it with the NRaas super computer found here.
http://ts3.tscexchange.com/index.php?PHPSESSID=3194b94ba7e492a8562d46f5ac8e14bc&topic=996.0 Just create you family the way you want them, set the teen as the child of the adult, and lave the young adult alone for now. After you have moved them in, click the parent sim, you should get a "Master Control" option. Under that menu there will be a few options and some more sub menus, click the "Advanced" menu, which will give you the option to change your family tree.
After you're inside the Advanced sub menu, click "Add Child", This will bring up the last menu you'll need to work with, it will list all of the town's sims filed into different categories, click either Gender:Male or Gender:Female, depending on the sex of the young adult, this will display all the ladies/gentlemen alphabetically by their last names, should be easy enough to find then, after you've done all of that. Check your family tree, it should have him or her in the branches now.
Sorry for the semi-rant/explanation, but I hope that explains everything, Good luck
