Once you launch the game and get the next window open, you'll see 4 squares above the check button. One of these will be your legacy family and the other 3 will be blank. At the far right, you'll see a plus sign and it says "New Game." When you click the plus sign, it will take you to the start of a brand new game and you'll start is just like you started your legacy family. When you save it, one of those 3 blank slots will be taken with it. Also note that even though you only see 4 slots, you can have many more than 4 games saved at a time. There's an arrow to the right of the 4th slot that will let you scroll to the rest of your games.
You shouldn't have to save a backup file of your legacy family if you start a brand new game the way I've described. Then when you want to play your legacy family again, just click the correct square for them and off you go!
I hope this answered your question!
