My tantrum sister

So I have a 22 year old sister who is supposed to be more mature than I am but every time something goes in my favor or even if it doesn't and she doesn't get her way she acts like a spoiled little brat. She acts as bad as my three year old nephew does sometimes. I swear I'm the youngest but sometimes I wonder who the most mature out of the three of us is. Urgh she really frustrates me. When I try to have a calm discussion with her if I even mention something she doesn't like she starts yelling and throwing a fit. She wants to act all depressing and negative and isn't trying very hard to grow up.i was at least out of my parents home for a couple months before having to move back for a little bit due to roommate trouble and then back out and living with my oldest sister now. Seriously when I do spend time back home it only stresses me out worse because she wants to argue about every little thing. I just want to know why she wants to act like a child instead of growing up some. I mean yeah I am still a little kid. I like watching family movies, or playing games, and swinging on the swing set, but do I act like a three year old. No. Why can't some people just learn to grow up?