Can you purchase life insurance on a family member in prison?

You can certainly apply for an insurance policy; whether the insurance company will choose to accept your application is up to them. A family member in prison is not supporting anyone, so there is no issue of replacing lost income, however, there could still be a legitimate need for an insurance policy to help pay funeral costs. I am not sure how elaborate a funeral you may be planning, but a $5000 funeral policy sounds quite reasonable to me.