Adult Levo Scoliosis

I have suffered from back pain since an injury in 1993 and have a compressed and bulging disk in the L4-5 area as well as osteoarthritis.  I have a family history of serious Dowangers (sp) hump, spinal stenosis, and scoliosis that develops after the age of 40.  My sister's spine completely twisted from scoliosis within 2 years and had corrective surgery at age 53.   Because of all this, I have regular care,MRI's, & x-ray's.  Recently, I was diagnosed with Adult Levo Scoliosis and the onset of mild spinal stenosis (I am 50).  In comparison of prior x-rays, my doctor said this is a new development. I have developed a >15 degree curve within 2 years.  Can you shed some light as to why this happens so late in life?  I can't find any real info as to what causes the scoliosis as an adult.