Is this credit card fraud?

My sister used her card on her husband's Amazon account. She ordered something for me and SHE put her credit card info in. She knows how much the item is and she is anticipating the charge. People (customers, not people working for Amazon) on Amazon are saying it's credit card fraud because it's not my brother in law's card being used on his account and that his account will be suspended. I've used my mom's and dad's credit cards in the past on Amazon with their permission of course, just like this time, with no problem. So how is this credit card fraud.