What do you do when you're too weak to bike or ski?

So, here is the situation: my sister went in for a CT scan, and everything regarding her scoliosis seems more or less fine. Her orthopedic specialist just prescribed some physical therapy, and that’s been the extent of it for years. However, there is a much larger issue. Every single time she lifts anything heavy, she is hit with nausea and starts trembling; even just riding in a car triggers it. Don't even get me started on cycling or skiing. She hasn't been able to do those things for years. If she bikes for more than fifteen minutes, she feels faint and begins to shake—the same thing happens when she tries to ski or carry heavy objects. She has seen everyone from orthopedists to physiatrists and neurologists, yet no one can provide a straight answer. They find nothing out of the ordinary, so they just send her home. What is the problem with that? The condition is steadily worsening, and her ability to function is becoming increasingly limited. One neurologist once suggested this might be linked to her autonomic nervous system, but he didn't bother to explain the connection or offer any actual advice. Does anyone here actually know what might be happening? If you have any insight or advice, please, I am asking for help.
Thanks