How the hell did I become a third party in someone’s relationship???

Long story short I left a comment on one of my guy cousins story just making a joke about the video and then he responded to me then we talked for a few seconds, and then I said OK I had to go, and then randomly his girlfriend went on his account and asked for me to record all of our messages because he deleted all of them so I did and she thinks that he’s cheating on her which I actually think he is because he showed us his girlfriend when we went to his house for a barbecue one time and it was not her, but I didn’t say anything bc I don’t wanna get Involved anyway then she gave me his number to text and see if it goes through because she thought he blocked her anyway he never responded to the text until like a day later randomly at 3 o’clock in the morning so then we talked once again for a few minutes and then I said OK bye because my boyfriend was on FaceTime with me and my boyfriend was getting jealous and annoyed with him Plus I didn’t want his girlfriend to get the wrong idea and then a day later my friend came to me crying over the phone because my cousin‘s brother isn’t responding to her. They’re dating by the way so I went and DM him on Instagram asking why isn’t his brother responding to her and then we talked again for a little bit only for his girlfriend to go into his account again and start responding for him and saying really random stuff and getting mad but the thing is it wasn’t even me texting him. It was my boyfriend texting him for me in my account as we’re on FaceTime and it was just so Awkward and today I checked my Instagram. He literally sent me a DM saying I’m still mad about this and a random picture that I didn’t even open like I’m so confused what did I do to get myself involved in this?