Your witnesses must be at least 18 years old. Under Michigan law, these people cannot be your witnesses: your spouse, parent, child, grandchild, brother or sister. Also someone who benefits under your will cannot be a witness. Your patient advocate and your doctor cannot be a witness. Someone who works for your life insurance company or your health insurance company cannot be a witness. Someone who works for a health care facility where you receive treatment cannot be a witness. Someone who works for the home for the aged where you live cannot be a witness. And someone who works for a community mental health program or hospital where you receive mental health servies cannot be a witness.