Slow Internet Question

Hello everyone in the recent months I have been having problems with my internet. I realise this becasue of my high ping in games. I normally get 40-60 ping on any US server, but now my ping has gone up to 100-140 on any server even locally. My friend who has slower internet speed and lives in Peru gets lower ping than me sometimes, on the same server. This happens for 3-4 days then my ping will go back to normal (40s-50s) for a couple of days then back to the high ping. I am plugged into my router that is right next to my pc and there are 2 other desktops and a laptop that are connected wirelessly to it but only one if them is ever usually on and it is my sister's laptop. I have Bellsouth DSL 1.5 (highest speed i can get in my area).

I try restarting my router but that almost never makes a difference in my ping. I also switched the wireless channel in my router and that instantly lowered it back to normal but that has only worked very few times. It is not just in one game aswell but on any online game. One thing i noticed is that it seems like my ping rises at night. For example I would get home at 3pm and my ping will be fine but when i log on later at night it is in the 100s. Also sometimes I will get on early with high ping and watch some streaming movies and when I would log back in at night the net is a lot slower with high ping. Is this high ping because of my sisters laptop? She usually has music downloading but not too much and browses the net everyday. Could a new router improve my ping? I have a lynksys WRT54G. Is there a way that I can limit how much bandwidth she uses? ( I pay for the internet ).

Please help me with this problem because I play on competitions and high ping like this is killing me. Thank you in advance and sorry for so much text.