Your sister has serious problems with her heart.
The valve (mitral valve) that separates the left atrium from the left ventricle leaks severely. This makes the pumping of the heart very inefficient because a lot of the blood in the left ventricle is not pumped through the body, but leaks back into the left atrium.
In time this back flow results in serious problems.
- The left atrium (LA) has to manage the back flow of blood which in time results in dilation of the atrium
- Severe mitral valve regurgitation also places an extra strain on the left ventricle because, with blood pumping backward, there is less blood going forward with each beat. The left ventricle gets bigger while trying to compensate for the back flow. In time it can not compensate for it anymore and becomes larger and weaker and the pumping force goes down. (dysfunction during systole and diastole and low ejection fraction (EF).
I am no doctor, but it seems likely to me that the cause of the heart problems is the mitral valve. If her doctor can repair or replace the valve, the heart might perhaps improve again.
Please ask away, if you have further questions.