I freaked out once with my first because one day after lunch I realized I hadn't felt her move that day and I couldn't GET her to move when I pushed on her, which was very unusual. I mean, I FREAKED OUT. And I'm not one to call the doctor about every little thing. In fact, his nurse reminded me several times that I could call if I had questions... I just take most things as they come. The baby not moving? That's a whole different story.
I also think I was about as far along as your sister is.
I went into the doctor's office, they hooked me up (they were SURE everything was fine, they were really just doing it to reassure me), and she was fine. It took about 45 minutes and me changing positions to get her to move a little, but it finally happened (they thought they'd have me out of the office in 15! - she was stubborn that day).
They suggested she was just sleeping. At that stage they're growing by leaps and bounds and they need to sleep, too.
Try to reassure your sister, but also be there for her. You never really know exactly what's going on in there until you get to the doctor.