I was playing a legacy challenge in a custom world and I had my founder ask his girlfriend to move in. She agreed, but when I moved her in, I saw that she already had two elder cats, was at level four...
All Sims are EA's property. There is no legality issue here or even any reason to worry about it - you can play it, share it, etc. Many talented people create worlds for Sims that have Sims with back stories - they are meant to be played.