Harry Potter and the potions accident.

...Explaining...

Ages

Doragon is Draco's twin. ~Died at age 2~

Hannah is Harry's older sister. ~Died at age 3~

Each of their ages are effected by their siblings current condition (age wise).

Since Harry is 17 like he's supposed to be Hannah is 19 like she's supposed to be.

But since Draco was deaged (to two) I multiplied 2x2 then subtracted the age of what the twins were supposed to be (17) and got 13

Draco is not physically the right age so Doragon won't be either, well maybe...

Deaths... 

Doragon was killed by Lucius, because she accidentally broke a vase.

Hannah wasn't going to died Halloween 1981, but she jumped in front of the killing curse meant for Harry and was killed. There was some sparks from the curse that caused Harry's scar, he was not hit directly.

**If there's anything else you want explained please tell me**