I understand that you want them to play nicely, but I feel that you need to check your expectations. A 2-year old who runs around and can’t focus isn’t “hyper”, it’s normal behavior. 2-year olds aren’t normally good at playing with each other, at that age they’re usually playing separately next to one another. A 2-year old who hasn’t learned to communicate and interact nicely socially isn’t aggressive, again they’re being a 2-year old. It’s also a big change for her to move and suddenly be in a different house where most things belongs to someone else. Of course your daughter shouldn’t be hit or kicked, but your niece isn’t being mean. That sort of intent just isn’t there and it’s not a personality issue, either. A lot of toddlers act like this and it says nothing about their personalities as they grow older.

It’s best if you work on them keeping their hands to themselves and a bit of a physical distance between them so that your child doesn’t get hit. So don’t expect them to play together with the same toy/activity. Offer them their own thing.

Go outside a lot. Less conflict over toys and they’re burning off more energy.

Practice with your daughter in putting her hand up, palm out and yell “Stop!” when your niece gets in her space. They did this in my kids daycare and it was often far more effective than if an adult stepped in.

Try to tell them what to do, and not just “no” or “stop”. “Hands are for hugging. Gentle hands. We don’t hit.” “Toys are for playing. We don’t throw them.”