help

I need help with my device. I think someone is monitoring my device and I don't know how to find out. But the reason I say that is because I can send a text out or an email the person knows stuff that I never told her. The people I talk to doesn't have her number and doesn't communicate with her. I tried restarting my phone but it she still sees stuff. One day I saw her downloading something on her phone and she asked to see my phone. We are family but now it's a little strange that she knows too much and I just want my privacy. How can I undo this so I can get my privacy back.