Sims 3 code already redeemed

@Zamp121  You can try repairing the game in Origin: open your game library, right-click on the Sims 3 icon, and select Repair.  If that doesn't help, clear Origin's cache, and if that doesn't help either, uninstall Origin "the hard way" and reinstall.

https://help.ea.com/en-us/help/faq/clear-cache-to-fix-problems-with-your-games/

https://help.ea.com/en-us/help/origin/origin/manually-uninstalling-origin/

If none of this helps though, your only options are contacting EA support or reinstalling entirely via disc.  For the support route, you could provide a photo of the disc, case, and insert with the serial code showing, and ask why the pack isn't working in your account.  (Your family could send you a photo, but make sure they don't edit it in any way.)  Support isn't always helpful in these cases, but you could always try with a second person if the first one couldn't fix the issue.

https://help.ea.com/en/contact-us/

Please keep in mind that support is swamped right now between its reduced hours and all the people stuck at home with nothing to do except play their games.  Phone calls are entirely unavailable, and live chat is only open from 9 am to midnight GMT, Monday through Saturday.

The other option, if you can't get anywhere with support, is to uninstall the Origin version of the game and reinstall via disc.  You'd need both discs, of course, plus the base game copy would need to be manufactured before September 2012; the newer ones require Origin to be present and managing the install process.  You can check the copyright date on the back of the case, although a 2012 disc could go either way.  It doesn't matter how old the Pets disc is.