<human>: I[26/f] am in a 5+ year relationship with my SO[29/m]. We've lived together for all of those 5+ years (dated after being roommates). His half sister[22/f] is very pretty, but very immature. We (her and I) have a very good friendship. HE and her also have a very good relationship (calls each other "babe", "baby") despite having a different father, and growing up apart (he moved out of the county when he was 17 & she was 8, moved back when we met).
We've (S.O. & myself) always been very trusting of one another. He uses my phone & computer whenever, knows all passwords, and I've never put up a stink about shared privacy. But the other day, he found- and forwarded- a text message of a mutual friend (of all of ours) to his sister that was meant for me only; describing that he, our friend, has feelings for her. SO claims that he is trying to protect half-sister, but doesn't care that he went through my phone, and hurt our friend's trust in me. I care about his sister, but the over-protectiveness is becoming creepy when his loyalties lie more with her than myself and my trust. So r/relationship_advice of reddit... what would you do?

Can you summarize the given text in a few sentences?
<bot>: boyfriend snooped through my phone & forwarded one of my conversations with someone to his sister. Thinks he did nothing wrong.