windows has something called AutoTune and i know this can get disabled.
i would run this on both your computer and brother's computer.
netsh interface tcp show global
To tweak AutoTune
netsh int tcp set global autotuninglevel=normal
and these are possible values
disabled : uses a fixed value for the tcp receive window. Limits it to 64KB (limited at 65535).
highlyrestricted : allows the receive window to grow beyond its default value, very conservatively
restricted : somewhat restricted growth of the tcp receive window beyond its default value
normal : default value, allows the receive window to grow to accommodate most conditions
experimental : allows the receive window to grow to accommodate extreme scenarios (not recommended, it can degrade performance in common scenarios, only intended for research purposes. It enables RWIN values of over 16 MB)
try the following as tests
- disable ipv6 and test
- have rclone bind to a ipv4 address using https://rclone.org/docs/#bind-string