Can she really check my messages online?

Okay my sisters always threatens me that she can check my messages and see what I'm doing ect,? Is that really true? I have a iPhone 5s on tmoble, my father and I are the only ones who have access to it, can she actually look at my phone history?

I mean it's technically true but you would need to be pretty good at hacking or have access to a stingray device. If she has access to a Mac computer that you backup your phone on she can see iMessages you send if you enabled that when prompted.