Hello,
Indeed as you mentioned, childbirth is the culprit in many sacro-iliac problems! Has your wife ever seen a physiotherapist who specialises in obstetrics, or one who is skilled in treating SIJ problems?
The reason I ask is because there are some fairly standard tests that show whether the SIJ/pelvis is out of line and the treatment for it can also be really quite effective and is not usually painful.
This type of problem is also quite common in people who are hypermobile - double jointed or very flexible - which might or might not be a contributing factor.
Sorry I can't be of much more assistance but see if you can find a physio as I mentioned above, it might be worth a try at least. They will also be able to help with regards to helping your wife cope around the house and in her everyday life - or refer to an Occupational Therapist who will be able to do so.
Ruth Cheesley BSc(Hons) MSc MCSP
Chartered Physiotherapist