what could be using up my internet?

[Patamon]

my dad buys alot of gig a month for internet, and the downloads hav gone in 9 days. my dad rang the internet people up and they told him 15 gig was used in one day. now the internet is used mostly everyday of the month, i play league of legends, to play a game on league of legends is a bout 10mb and i play about 5 games a day so that that would be 50mb everyday. my brother though constantly watches videos(porn,youtube,megashare) and downloads movies, music and tv shows from itunes and stuff. my sister just uses it for facebook and twitter and stuff and me and her proballly would only watch a youtube video once a week or so. so who do you think is the one using it all up!? please help. my dad is taking away the internet from all of us and my brother is probally the blame for it since he is constantly downloading stuff