Sciatic pain in your scaro-illiac joints is common in pregnancy and I'm really sorry to hear it giving you so much trouble. Keep up with your stretches and try to tighten your buttock muscles at the same time. Strenghening these muscles=more support for your your lower back.

Also if you have not already, try lying down on your back, bending one leg over the other and hold onto it with the opposite hand. Then gently lower your bent leg to the floor, bouncing gently. Try doing this on your bed if it hurts on the floor.

Hope it feels better for you soon.