I take it that because you have access to the internet that you are on line.
Like has been said earlier VoiP is the way to go, although I think calling the US is typically one of the cheapest places you can call on most plans. £10 for 50 minutes is pretty steep, even some of these calling cards are offering 2 or 3 hours for only £5.
I take it your sister is also on line, so the best option is for the two of you to sign up with
Skype, I used it whilst down under in Singapore, Aus, NZ & Thailand in August and my 6 hours worth of calls to some land lines and mobiles costs me in the range of €8. (£6.20 @ 17/10/08 rate)
These days it really is cheap to talk
