I currently have an iPhone 4S and plan to switch to my sister's old iPhone 5. The sim cards are different sizes and I have to order a new one. If I order a new SIM card for the iPhone 5, will my SIM card with my iPhone 4S still work in the meantime?
Yes. Until you activate the iPhone 5, either online, on the phone with CS, or on your own, the current phone will remain the active phone.