Go into your router and shape the allocated bandwidth to their PC lol... And don't say anything.

Look into QoS for your router.

As others have said, probably best to just enforce at the router level...

Before anyone goes to war, it's probably worth even clarifying if it's true or not. I had a roommate convinced I was "hogging bandwidth" but our internet was just shitty. Over copper. Very inconsistent. I had to literally shut off everything i had to convince them it wasn't even me.

They swore up and down it was me. There was a pattern. It was the only explanation.

Talk to them about it like adults? But I gather these are children, so I don't know.

I am the network. If you're hogging all the bandwidth I'm knocking your device's priority down and maybe even gating it to 1/2 or 1/4 the total bandwidth. If you cant manage the shared resources respectfully it will be managed for you.

Get a better router. Once my kids started gaming, I started lagging bad. The cheap routers just can't keep up.

Maybe talk to your folks about adjusting router settings. That way you can avoid confrontation by going straight to your parents. 

Deauth them off the WiFi repeatedly for the shits and giggles

Throw a slice of cheese on their head.

I mean, it can't hurt, and it'd be funny. Also, if you post video it might go viral.

Router QoS. Set the router to prioritize your traffic based on IP. What are they doing that's consuming the bandwidth? The only times it's been an issue in our house has been when I've switched media with no speed restrictions. Past large and fast downloads, if you have high speed internet, the pipe shouldn't be congested. Otherwise, be confrontational then escalate if behaviors don't change.

Ethernet cable

Hog it first

kill.

I move out of the early 2000.