How should i manage my sister?

So I usually take care of my sister and I basically taught her pretty much almost everything she knows.

I'm 18 and she's 8. But I told her to sit down due to her acting up and accidently hurting a sibling. I wasn't going to have her seated for a long time. But anyways she got up and took her phone from the table.

I told her she couldn't play with the phone for now and she still stayed on the phone. I took the phone changed the password and while I was doing that she kept asking what I was doing. I told her to stay seated and she kept getting up trying to see what I was doing.

Changed the password and she cried. She stayed seated after that.

I told her "you can't have the phone now because you kept getting up after I told you to stay still. You need to listen."

She was mad and crying but not screaming or any of that. Now the phone is taken away and she's playing with a sibling.

Idk if I was wrong or what to do. What do you think? Idk how to manage a child much.