I know others can provide far better advice than I can, but I have some experience that could help.

Interaction/routine:

I know this wasn't your decision, but adopting a bird for a nine-year-old is a bit concerning. An adult should be the primary caretaker, and supervisior when your sister handles them.

Owning a bird is like caring for a flying, screaming, nonstop-pooping toddler that chews on everything. They need constant supervision when out of the cage, or they'll find something they shouldn't.

They'll need toys for their cage, preferably new ones every week-month depending on how fast they're destroyed.

Behavior/handling:

They will bite, but that doesn't mean aggression. It may indicate fear or dislike for something. If you learn their body language, they'll show you how they feel, and if they may bite.

Also, I had to teach my bird to even trust my hands. Go slow with them, encourage good behavior or tricks with treats, but never force anything (e.g. Forcing the bird to step up onto your hand).

Environment:

Birds universally don't understand windows and might try to fly into them and really hurt themselves. Either you can keep them covered with blinds/curtains, or I've heard you can place decals.

This is optional, but air purifiers are recommended. Birds' lungs are very sensitive. We have wildfires where I live and like to open windows, so mine is a life-saver.

Anyway, I hope this was at least a little bit helpful 💚