Is post mortem mandatory for death by disease?

My aunt age 52 suffering from Gangre, improper blood circulation , heart problem, diabetic.Also she is suffering from Scleroderma (its a disease where your anti bodies start attacking your own antibodies). She is bedridden, but can speak, read , see etc..She is surviving mainly on medicines and visits doc monthly twice. If she dies, do we need to do post mortem? Is it mandatory if she dies bcoz of theses diseases?