How to make my conversations private?

Ok so i have an iPhone 4s and so does my little annoying 13 year old brother. I have a feeling that he somehow goes through my text messages through someone else phone.somehow he knows my conversations and i know that he can't get it directly through my phone. Is there a way i can out a stop to this. Any privacy settings. Please help. I'm 18 years old and he is only 13 and i have no privacy at all.

I don't think it's possible for him to do that.