Weird issue: One PC on the network effecting all other network connections

[HeX]

In my house we have a bunch of PC's, tablets, phones, consoles etc.

We have a ASUS RT N66U router which handles all the wired connections and provides WiFi.

Hooked up directly to the router are:

My PC
Wifes PC
Xbox One S
Powerline Adapter

Via Powerline fed from above:

Daughter 1 PC
Daughter 2 PC

Then a multitude of other devices via WiFi 2.4 and 5. All PC's running Windows 10.

OK, so here's the issue. Everything works great up until the point Daughter 1's PC attempts to download ANYTHING. The moment a sustained download begins (like from steam/windows store/epic store etc) all other devices either lose connection or the internet speed is throttled down from 150mbps+ to ~1-10mbps.

I've reset the router a number of times, there's nothing as far as I can tell that should cause this. Even setting up a QoS setting that limits D1's PC to say 40mbps has no affect. Her download might only be going 5MB/s but all other internet in the house dies.

Downloading on any other PC doesn't cause this issue.

Just general browsing and gaming doesn't cause the issue.

I've tried 2 different powerline adapters, I've tried disabling the Ethernet connection entirely and using WiFi, same issue...

I've tried using DHCP and static IP addresses in case of conflicts, same deal.

Short of nuking it I'm running out of things to try, I just can't see how it'd be a software issue when it affects the whole network?...

Any suggestions greatly appreciated!

 

[Quartz]

Even setting up a QoS setting that limits D1's PC to say 40mbps has no affect.

What happens if she tries to download over a wired connection?

Can the powerline adapter that is connected to the router cope with two upstream adapters? Or are both daughters' PCs plugged in to the same adapter, perhaps via a switch? In that latter case, try putting a router between the PCs and the switch.