The easy answer is get a mod. Twallan's Story Progression is the go-to mod for getting your town's cogs running (refer to our Game Mods board if you have any further questions on it, though).
Of course, if you don't like mods, you can do all of the dirty work yourself. Switch households via Edit Town. Set up some relationships and pregnancies, and then go back to your family and watch the results. While it involves you doing the work, there is a slight bonus of putting together couples and situations that you specifically want.