Have you tried another known working at speed ethernet cable?
Is your PC directly connected to the router or are there other components: ethernet wall outlets, switches, a second router, etc..
Are you able to switch your ethernet connection to your brother's computer and vice versa?
Determine if the problem follows your computer or stays with your connection.
On your computer try running "ipconfig /flushdns" (without quotes) via the command prompt.
Reference:
https://www.lifewire.com/what-is-a-dns-cache-817514
Just a bit of "house cleaning" to eliminate possibilities.
You mentioned ping. Have you tried pathping and tracert? Run them all at various times targeting Google's website and the game site(s) you play.
Post what you identify as being representative/average results.
Someone may be able to spot a potential culprit.