My sister is experiencing aching muscles on her legs. she walks a little bit and it starts to pain just below the butt and downwards about a week now?

Nerve pain: This sounds like nerve rather than muscle pain. there could be some inflammation or infection going on as well. I would recommend you visit your doctor for a proper examination and possibly X Rays or ultrasound of the Lower back , hips and thigh

Muscle or pinched : There are are some things that we would consider. It could be a pinched nerve from the back which sometimes can be called lumbar stenosis. There could be a muscle problem. But in either sending you needed an evaluation by your physician