Dr Loren Fishman has studied the effects of yoga on osteoporosis (you can google this easily ,and it comes up on NIH site), as well as conditions like frozen shoulder, and has found significant bone benefit to yoga.
Having said that, I've done yoga for years, as well as weight lifting, running, swimming, biking (triathlon and training for triathlon), and I've just been diagnosed with fairly advanced osteoporosis of non-vertebral bones, including the radius of my dominant arm. SO frustrating!