What causes GD?

Just kinda puzzled why some mummies have GD while some don't? Asking this because I'll be gg on a holiday with my SIL. Both of us are pregnant, I'm on 28th wk while SIL is on 24th wk. She was just diagnosed with GD while I don't. Weirdly I'm the one with the super sweet tooth and she has been the healthy one, exercising and taking care of her diet. I've been craving for teh tarik almost everyday since 1st tri and I never curbed it but my SIL hardly eats sweet things but she has GD! So weird! Not sure if this is genetic cz my fam has no one with diabetes but my late mother in law had it. Or does it also depend on our carb intake? Cz I'm someone who hates rice while my hub's family loves it. I only eat noodle/pasta occasionally as I'm more of a meat eater. But I can never ever resist a sweet dessert/drink. It's gg to be a holiday with lots of food and I'm gonna feel so guilty if I were to eat in front of her. Hmmmm