I said in the post I have tried the pc at my bros house .
Have you run the waveform test for bufferbloat?
Ideally run a number of separate tests both from your connection and your brothers connection( if that is working as it should).
Did you get anywhere with other loaded latency ping result tests, as noted above?
Can we actually see your BQM result (and again that of your brothers for comparison)?
You need some actual useful collateral if you’re going to try to take this forward - perhaps to swap VLANs if that there is indeed a congestion issue.