How to monitor upload/download on multiple machines [migrated]

There are 3 windows machines on my home network. I'm hard wired to the router whilst my parents and younger sister are on wifi.

I've noticed the occasional drop in connection speed recently and I have a sneaky feeling my 10 year old sister is doing some ridiculous things on her laptop that's probably killing the network speed.

Is there a program or app that I can use to see what pc is using the most bandwidth at any given time?

Thanks