Is too much sugar related with diabetes(please read)?

[CupcakeHater]

I am NOT talking about the link with obesity cause I know obesity may cause diabetes,but how about being on a normal weight(I'm worried about my 21year old sister)and eating HUGE amount of food,especially carbs and sugar(I mean HUGE like she eats all the time,everytime her plate is empty she fills it again and eats while being busy watching tv or using a pc).As I know she's weight conscious,when she'll just get 10pounds heavier(she still wont be obese,but slightly overweight)she'll slow down and realise(like she ALWAYS does).But is it possible she has or will have diabetes BEFORE she gets obese?(because of the food).My GRANDMOTHER has diabetes(the type where she needs to get insulin shots everyday)but she got it at an old age and after all she's quite old,so I don't think it;s the "genetic"kind that'll pass to my whole family.