Having family history of diabetes(grandparents) does it mean there is a high chance you are going to get it? or can you avoid it with a healthy diet?

Can avoid: If you are good about maintaining a healthy weight, eat mostly fresh, unprocessed food, and exercise, your risk will be greatly reduced. Try to eat sweets and simple ("white") carbs rarely. Whole grain type of carbs like brown rice are better for you. You can get a rough idea of what a good weight is by finding out your bmi. It should be 23-27 range.

Increased risk: You have a greater chance of developing diabetes later in life especially if you gain weight, and do not get adequate exercise. Healthy lifestyle will be very important for you.