"Both of them" (if they are each a doctor) or "neither of them" (if no one is a doctor).

I have two sisters. 1. One of them works as a doctor. 2. Neither of them works as a doctor. 3. Both of them work as doctors. For more than two: I have three (four/five) sisters. 1. One of them works as a doctor. 1a. Two (three/four/some) of them work as doctors. 2. None of them work as doctors. 3. All of them work as doctors. Just learn it that way. Watch out for singular and plural use.