Relationship trouble: boyfriend's brother and friend?

I have run into trouble with some of my relationships. I will try to describe the situation as it happened without letting my emotions influence it. Around 12 at night my boyfriend and his older brother went to pick up something from Kate's* house. My boyfriend and I were already in a minor argument before this. When we were on the phone arguing, he told me that he was still in Kate's house, this was around (1:30am). This was not ok with me as I found it disrespectful that he did not step outside and made this his brother's and Kate's business. His brother was texting me from Kate's phone saying that I needed to calm down and that I was being childish. (This upset me, my boyfriend does not know about this. ) They did not leave until I told my boyfriend they needed to leave her house right now, and he needed to talk to me on the phone alone with no people around. We resolved our issues which stemmed from the fact that him and his brother cannot do anything at normal hours. I have expressed myself to him and my boyfriend is a saint when respecting my wishes. He does not go out late at night whenever I am not invited to events (as we all have the same group of friends) since if the situation was reversed he would be upset. This was not one of those situations but it falls along the same line. We made up.
However, the next day Kate sends a screenshot to his brother that I had sent her showing that we had resolved our problems. The screenshot of the text read "Boyfriend: I promise to listen to you more, I love you" "Me: you better" "Boyfriend : I do". His brother and her found this to mean that I was belittling my boyfriend and in a way they all agreed it was me "making him my 'b***h'". His brother showed him the screenshot and my boyfriend was enraged with me. I am upset by their actions as they have caused my boyfriend to become highly upset with me as those were never my intentions at all. After a very heated discussion, I finally got to show my boyfriend the context. He understands I did not mean to disrespect him but still feels disrespected just the same. We made up, even after he said he would end things, and this situation caused us a lot of stress. I am asking for help on how to clear the air with Kate and his brother. A little more background, out of frustration I have texted Kate that it was not her right to send this and that she should reflect on it. Kate has only met my boyfriend's brother only once before this. His brother is around 23/24, we are all 19, and he has friends in our circle. She became upset and said she sent it because she feels that I need to stop being a "sh***y" person to my boyfriend. I always discuss my relationship problems with her and this is probably why she feels that way. Thank you.