This applies to Sims, Pets, and Houses. People can be protective of their content, and many will not put their Dynasty/Legacy characters on the exchange for that reason. Items you download here have been shared with this community to be used in games for entertainment. It does not mean you have permission to use them in a story. Get permission first or a complaint from the creator may result in your story being tossed into the graveyard.
If you've unintentionally done this, tell them of your mistake and solve the problem. A person discovering on their own that you've used their dynasty characters in your story are more likely to be upset and file a complaint.
Best practice: be creative and make your own characters.
