This is an answer maybe only one person can know right now. There are many things that can cause this to happen. A blood test being positive is only the first thing any suspecting person looking for causes into reasons she feels the way she has been lately too. The very best diagnosis is by a liver biopsy. This is done on an out patient basis in a clinic where the radiology doctor goes into the liver and takes a very small piece of the liver for the lab to examine. The liver is the only organ in our body that can rejuvenate itself too, which is great. If the cause was through something that might be contagious, like any Hepatitis, or disorder like that than yes, without treatment it can be very serious. If it was caused by something other than a permanent disease or disorder then no, it can be helped with the treatment and advise of a GI doctor. This is their specialty, all the digestive disease organs are. Your sister knows or should know whether she's been exposed to anything contagious and to follow up on it too. If so please ask her so that you can try to make sure you don't catch it from her. Another thing is to make sure your mom or guardian knows asap. This is to protect you for the rest of the healthy life you deserve. Remember the liver does and can rejuvenate itself, so make sure the earlier you know, the sooner you'll be better, and your sister is the only one that can start the ball rolling. Good luck and God Bless
nursing and personal experience
I caught Hepatitis C as a young nurse from a patient. That was how I got it and followed up only after the patient felt guilty enough to tell the hospital.