I recently got a router. Up to now, it's been working fine, even if my sister's using the internet on another computer. Now, however, it slows down to a crawl if there's someone else on the network, and even when people aren't it sometimes slows down randomly. Is this because of my internet plan (it's quite large and I don't surpass it), my router or something else?
For my computer I use a wireless USB stick thing made by TP-Link, and my router's product number is TL-WR1043ND.

It could get slow because someone (outside your household) is using it. Do you have a password on it?
-----
You can also go to the customer service for the company that created it.

Here is the basic operation for your product. See "wireless security".
http://pigtail.net/nicholas/tp-link/index.html

Wireless security is enabled. It's WPA2-Personal.

Sometimes mine will just cut off for a sec and cut back on. Most likely because theres is more than 3 people on the internet. My router sucks.

As for your router, what are you doing when it slows down?

maybe your sister is downloading too much
or maybe your mbps is too low

Nothing out of the ordinary. It just seems to slow down heaps/disconnect entirely. But, after that happens, it stays slow for aaaages.