So sorry to hear they are on this path now
My DH was diagnosed Type1 Diabetic when he was 13. The JDRF have a children's and parent's magazine, i think it's called Countdown? They have loads of helpful info! My DH has an insulin pump instead of having to give himself needles or timing meals and watching sugar intake. It has made SUCH a difference to his quality of life! It's basically the size of a pager that he clips onto the waist of his pants, and after testing himself, he just adjust the amount of insulin that is pumping into him. He can eat whatever he wants, drink whatever he wants, at any time of the day so long as he keeps the on top of how much insulin is needed. If your sister's DD has the option to get a pump, go for it!