How am I going to limit our wfi users' bandwidth usage?

[budubidop]

We only have 20 mbps of internet, and there are 5 users here. My brother always keep on downloading and streaming, and i cant even use the internet to play online game. 

I want to limit the internet usage to 2 mbps. 

[Dan Lukes]

We know nothing about devices you used to craft your home local network. Thus - read product documentation is the only advice available. 

[budubidop]

we use Cisco DPC3825, it's a modem/router, sir. 

[Dan Lukes]

DPC3825 is an internet service provider (ISP) supported product. It's configuration and even feature set supported is customized for particular service provider.  You need to contact your ISP that you purchased this from to help you with your question.

A suspect the DPC3825 doesn't support the feature you are asking for, but only ISP mentioned above can provide authoritative answer to you.