Tell your brother-in-law to keep his grubby mitts off your computer in the future ! And it might be worth reposting this question in Technology ... there are some real experts in there. But include which version of Windows you are using and the version of IE as well (you can get that by clicking on Help, then About Internet Explorer, and you only need teh first digit of the version number not the full thing).
It is possible that he installed Firefox ... and that he made Firefox the default browser atthe same time. So, if you don't want to use it, uninstall it. that should leave IE as your browser by default as it were.
On version 6 of IE running under XP, clicking on Tools, then Manage Add-ons brings up a list of the add-ons being used by IE. The list will show you if they are enabled or disabled. Click on each one in turn and then click on the Enable button somewhere towards the bottom of the screen. When you have enabled everything again, click on OK then try the Cbeebies site to see if it will work properly. If it dosn't ... I'd say ask again in Technology.