Nsb25, I have O2, and I'd rate it as a good service. Lately I have had issues with O2 dropping my calls randomly, but up until this started happening I've never had any issues.
If you call your sister, she will not pay international charges, since it is an incoming call. However, for her to call or text you will be very expensive, unless you have the 'call me number', which is probably a better option.
I used to have an O2 bolt-on that allowed me deeply discounted calls to the USA. But now I mainly use skype and apps.
If your sister has a smartphone, and you have one here in the UK, there are a lot of apps that you can use to call and text for free internationally using data and WiFi. Some examples are Whatsapp, Viber, Kik, etc. That's what I use to stay in touch with my family (my sister and I are best friends too).