I think your OB is doing a good thing not relying on just an xray alone as a reason for you not being able to birth your baby vaginally.
You pelvic ligaments are soft and they stretch to accommodate the baby. It really is an unknown just how much it can open. Also the position you birth in has a lot to do with it ie squatting opens up the pelvis a lot more than being on your back. Ask your mum and your sister what positions they were in.