SI-joint/ and this client

I began working with my client in October, she is athletic and very slim other than her tummy that was saggy and hanging down, anterior pelvis with a rotation as right ASIS was anterior to left. She had tried exercise to tone her tummy but everything affected her lower back/sacrocilliac areas. Three c sections, children are 6, 5 & 1 the youngest is a very big boy that she carries around too much! lower back problems began after first child and had treatment from physio and osteopath. She has been attending weekly group Pilates and private sessions 2/3 per month. Pelvic stability is much better and she is feeling much stronger, abs are also looking more toned. All good other than the pelvic instability issue is still a puzzle, she quite clearly has unstable SIJ still as some side lying exercises cause a shooting sensation down the back of the leg, apparently not nerve muscular, the exercise was doing side lying on reformer in turn out with heel on foot bar,, right was good activated glute nicely, could not do on left. Although pelvic stability is loads better she still struggles to engage lower abs, I sometimes work with a cushion under tailbone. She also finds it really difficult to stand on right leg in a balance, this I found confusing as the right is the stronger, however she was leaning into the right leg which made it impossible to sustain, I did identify this but she was not able to overcome.

I hope this is enough information, i can't seem to work out what is going on in the pelvic area, how do I properly identify where the weaknesses are and where the instability is. How would you work with this lady?

Thank you in advance.
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Looking forward to hearing from you soon.