If I may, I assume that this is a rather large household/family with plenty of belongings? items in the house etc?
I have seen this problem a few times and let me make 2 suggestions:
First, if you have gardeners, and LOTS of things in the fridge, move all of it into someones inventory before copying.
Second, if you have an inventor, move all the scrap into someones inventory before copying.
This applies to almost all containers within the house that have a lot of things within it. It seems like the game takes a different type of inventory (count) of the items that are in large stacks within containers then they do of the sims and their personal inventory.
Another possibility is if you have a lot of items you can move them into the family inventory manually (the F2 buy screen household inventory, 3rd tab down). Doing these things speeds up the processing of the house considerably because it seems like the game gets to an item and stalls on it for some reason and isn't able to continue.