Pupils being different sizes is something you check for in first aid, it can be a sign of head injury or brain damage.....which would explain a LOT about Bella


unequal pupils
"Causes
- Aneurysm in the brain.
- Bleeding inside the skull caused by head injury.
- Brain tumor or abscess (such as, pontine lesions)
- Excess pressure in one eye caused by glaucoma.
- Increased intracranial pressure, because of brain swelling, intracranial hemorrhage, acute stroke, or intracranial tumor."
Maybe she got brain damage when she wrecked her car and killed her younger sister?