Most if not all ISP's cap bandwidth (basically, without being technical, it is how fast you are able to download something). Likewise, they also cap the amount of data you can download, known as a download quota. I assume you're more worried about the latter.
There could be a 1 GB file on the web. Whether it takes you hours to download it, or five minutes, it is still 1 GB, and it is that 1 GB of data that your provider is deducting from your monthly download quota.
So, limiting speed may help but only because it will annoy your siblings so much that they give up their particular habit of "movies and serial." Usually, bandwidth caps are handled by the ISP via the modem.
You'll simply need to explain to them that those things are simply off limits. Well, there's a solution for that too, but it's not so subtle. Block the website. There are several ways to do that. One could be adding the website to the HOSTS file (C:\WINDOWS\system32\drivers\etc\HOSTS if I recall correctly). They'll never find that. But, will they tolerate it?