@JackWCJones 

May I know do  your brother's pc and your computer use the Wi-Fi of power line device at the same time, to help isolate the issue, can you connect both devices to the router Wi-Fi nework and see if there is high ping or delay on router's wifi network? 

• You could try a different Wi-Fi channel and lower channel width on powerline device Please log in web interface of powerline unit by following instruction

Scan nearby wireless channels using software like inSSIDer or WiFi Analyzer and select a less-crowded Wi-Fi channel on the router.

Use lower channel width 2.4Ghz -> 20Mh, 5Ghz -> 40Mhz to reduce interference.

• Check if the high ping occurs on certain device or all the device, try a phone / laptop to test  ping, if its related to certain device you could consider update driver for the adapter. 

The downloading especially large files may lead to high ping, it is recommended to close  the unnecessary apps or download file in non-rush hours. 

Here is a relared FAQ for router high ping which explains some factors that could cause a high ping : What should I do if I get the high ping value from TP-Link Wi-Fi router?