Welcome to Just Answer. An MRI is better at seeing soft tissue than a CT scan is. The MRI will double check that it is just a cyst, and further evaluate the tissue of the liver to check for fibrosis, fatty liver, etc. It isn't necessarily anything different than what you and your sister went through, it's just that the CT is not as good a tool as the MRI when evualting soft tissue. Sometimes the CT sees something that looks like a cyst and the MRI makes sure.
When you perform a test, your aren't "looking for" any disease in particular. You are looking to see if anything is there. Evidently the CT scan showed something that looked like a cyst, but it wasn't clear enough for a firm diagnosis of cyst. The MRI is going to check that.