@JannaOneTrick, I am assuming that you want to play with this family as they are in your game but in an entirely different Save Game than the one they are in now (and not just that you want to move them to another house or another world or neighborhood within the same Save Game)?🤔 If I'm correct in understanding you there here is the way I've done it before:
1) Load your current game with the family you want to copy and when you get to the Map Screen that has their house on it, click on their house, but do not enter.
2) In the bottom right of the map screen with their house selected, instead of clicking on the Play arrow button like you normally would, click on the "More" button (button with 3 dots)
3) There then should pop up 3 more buttons to choose from -- click on the left one that is called "Manage Household" (button with people on it with a pencil)
4) Next a new box should pop up with your entire household's picture in it. There should be a line of buttons. Press the one in the middle that says "Edit, Add, or Remove Sims from the Household in Create a Sim" when you hover your cursor over it. (It looks like a pencil)
5) From there it will take you into CAS where you can make any changes (if you want) to their looks , or you can keep them the same and press the "Save Household" button in the top right corner. (looks like a folder with an arrow pointing in it.)
6) Then It takes you to the screen that shows your household and lets you take their portrait if you want . From here you can click on the "Save Household to my Library" button on the bottom right of the screen (Button with an arrow gong into a tray). You can also click on the button on the left side to save you household to the gallery if you want, but this isn't necessary.
When it is finished saving you can click "X" to get out of CAS and then you can exit out of the household management screen.🙂 Then you can Exit to main menu and from the Main Menu you can start a new game.
That was how to save your family to your Library so you can use them anywhere. Here is how to place them in a new Save Game: In the Main Menu, click on the "New Game" button. It will put you in CAS with a Sim as placeholder. Ignore that Sim and in the top right corner click on the "Open Gallery" button (looks like two photographs) and click on the "My Gallery" tab (on the right) and all of your saved households should be there, to place in any world you want! Click on the family you want and choose the option to replace the Sim that is the placeholder in CAS, and your family will be in CAS ready to enter into the home of your choosing! 🥳
I hope I explained this well enough. Please update us, especially if you are having trouble!
Happy Simming! 😇