You could do side bends to strech the muscles in your sides. Also, you could try to do sitted reaches. These two streches will help to strech your iliac crest.
The iliac crest is a bone, therefore you can't stretch it. but there are muscles around it that can be stretched. U can stretch the piriformis by crossing your leg and bending forward
Good stretches for the iliac crest are lunge and the butterfly. The butterfly is when you sit down and press the bottom of your feet together, making a diamond shape.
