It appears to be that your sister is habituated to consuming raw rice. This can be prevented by firstly avoiding raw rice lying around in the house. Give her cooked rice well flavoured so that she may forget about raw rice. She may try alternative like chewing gum or liquorice
sticks as their consistency may engage her curiosity similar to the experience with raw rice.
Usually gall bladder
showing crystals indicate growth of possible gall bladder stones
. Do get her blood tested for cholesterol
. Rice does not show as crystal as it would be digested in the stomach by acids.
If the technique does not work, kindly do approach a Psychiatrist
and Behavior therapist who will be able to evaluate her and decide proper management of this problem.