What is wrong with my niece she is a pain in the ar**?

For the past twelve weeks my sister and her 6 year old daughter and her 2 year old came to live with us because my brother in law left my sister because they wasn't getting along. My niece is such a pain in the ar** she is rude and cheeky, she doesn't do as she's told, she is really hyperactive, she always goes in my room and she goes through my stuff and figets with them and doesn't leave me alone in peace, she even gets a bit aggresive and she throws tantrums, she is also very nosey and listens to peoples private conversations and sh** stirs and cause arguments by telling her mum about the private conversations, she is also very greedy and always says can I eat this and whenever we are eating she always asks us if she can have some or have a taste dispite after she has had her own meal, and she is also very bosy like she says can I play with that, or can you take me to the park or can I watch this constantly everytime and everyday. And whenever I tell her off for being rude her mum in other words my sister has a pop at me, my mum and my older brother by saying leave her alone she is just a kid and know we are waiting for her to get her own place because her friends sister has a house that she cant rent and she cant get a house anywhere else because she is claming dss.