The Sims 3: Tw?

http://www.the-isz.com/nraas/index.php?showtopic=918 You can use this mod to allow relatives to have relationships. Just go under "Romance" and make sure "Allow Near Relation Romance" is set to true. If you don't want other related sims other than your twins to be romantic, make sure "Allow Autonomous Near Relation Romance" is set to false (which is it's default) that way no related sims will have a relationship unless you tell them to. Other feature included in the Mod are on the page.