Update your post to include full system hardware specs and OS information.
Look in Reliability History and Event Viewer for any errors, warnings, or even informational events that started just before or at the time your brother played fortnite.
Run "ipconfig /all" without quotes via the Command Prompt.
Post the results.
= = = =
Make and model router?
Your network router may also be capturing some error or related problem information if the router supports doing and is enabled to do so.
Who has full admin rights to the network router? You will need help from that person.