This can be a hard area to stretch since there is not much motion in the joint to begin with. Here is the best stretch I recommend for a little relief when the sacroiliac joint is really irritated. Cross the other leg over the leg on the affected side, and relax forward. Hold that position for 10-15 seconds, then stop. Be gentle. You are trying to wring inflammation out of the muscles... https://www.french-chiropractic.com/sacroiliac-joint/