Are there good twin studies on whether or not alzheimer's is hereditary?

Yes. Alzheimer disease exhibits multifactorial determination with many genes plus environment determining who will be affected. Twins and siblings have a slightly higher risk unless it is a rare case where single gene mutations are involved. These families exhibit early onset of alzheimer and relatives of affected persons have from 25-50% risk (see http://www.Mayoclinic.Com/health/alzheimers-genes/az.
Yes. There are twin studies as well as other studies that show alzheimers risk is partly hereditary. Early onset alzheimers disease with onset in 40's or 50's is highly inherited while late onset disease has a smaller genetic component of risk.