No offence but personally, if that's the way your live, finding another woman would likely be on my agenda too. It's a message for him on his phone, yet you just up and read it..doesn't matter if it's your sister, or not as it turned out. Maybe i'm just a bit funny like that, but i expect my right to privacy especially if it's nothing to do with you whatsoever. Couples seem to get funny like that, they think that they are entitled to know anything and everything without any prior agreement. My emails, my texts, my call history, it's noones fucking business unless i want it to be. You did say you already caused 2 breakups already, as was said earlier, maybe there's something about YOU that is causing these deviations.