Having a physician who is aware of the potential adverse effects of certain psychotropic medications is very helpful. He or she can monitor drug levels and bone mineral density (BMD). Periodic testing of bone density every one or two years may be advised.
Good nutrition and exercise are also very important. If your sister smokes, that can be an additional risk factor. A calcium-rich diet along with calcium and vitamin D supplementation has been shown to help.
It may be helpful to consult with a nutrition counselor to get the best advise for your sister. Her age, general health, weight, and current medications will be considered in planning the best program of prevention. Speak to her doctor about your concerns, and ask if a referral might be a good idea.