What should i do

my sister is always hiding secrets from me and keeping stuff behind my back what should I do

Disco, Have you found out any of the secrets? if so are they bad ones?

honestly, I think you should just leave her alone
if she wants you to know she will come to you
me and my older sister are pretty close but if one of us has a secret that we arnt willing to share we just leave each other alone about it, plus, it usually comes out later so dont worry about it
and dont try to squeeze it out of her because that will just make her not want to EVER tell you stuff

Honestly, it sounds like your trying to be the mother of your sister. I know its hard, even if you are just looking out for her. It really depends on the secrets your sister is keeping from you. If they are bad and may harm her you may want to
A) Sit her down and talk to her about it (be nice, dont raise your voice or threaten)
B) Mention it to your parents
C) Get her friends to talk to her about it

I have 2 sisters, one is older than me and the other one is younger than me. My older sister ALWAYS keeps secrets from me. I really cant do anything about it but she ends up growing to trust me and then tells me anyways. You just have to show your sister that she can depend on you. Keep the little secrets that she tells you, help her out with out telling on her. It works 3 Good luck