Depends on your router. What are you using?

There're a variety of ways. I'm using Mikrotik so I can do something like:

1. Set 2 queues. All ethernet connected devices to q1 and others to q2.

2. Set a speed limit for wifi so everyone using wifi only gets 10mbps.

3. Even simpler, limit your sibling's device speed to 10mbps. and set yours to infinity.

etc...