I (20 F) have a younger sister that is 17, to give context to the situation my sister (let's call her K) met this guy (We'll call him J) at a college she was taking courses at when we used to live down south. After we moved out of state this past year to a small town in the middle of corn crop nowhere, I discovered that she was still talking to him at the public library online. As time went on I found out that he was my age! I wasn't too keen of the idea but didn't say anything, because I live in a strict household where dating is only discussed after we move out, and knew that she needed someone to talk to. things began to progress between them, to the point that they were practically dating online but said that "they weren't" until she was older. My Younger brother who is older than her even worked with J to send K a phone in the mail so that they could call and talk. this became an immediate red flag in my mind because the only way us kids were allowed a phone was if we were working and paid for it ourselves. I warned my sister that if she was not careful with it she would get caught, and though she said she would, it wasn't 3 months before my mother discovered the phone and was furious. She confiscated the phone and tried to search through it to find out who this guy was. since my brother was also friends with J online, he quickly called J and had him discontinue the service and lock the phone (because it was in J's name) when it was revealed that all of us siblings knew about the phone and didn't tell her, she scolded us for 2 week straight over and over, telling us how it was illegal for a guy his age to seek relations with K (She was 16 at the time) I did not know this, but once researched the facts myself, I unfriended him on social media and deleted his number from my phone so both him and I would not get into more trouble. Fastforward a few months, me and my younger have our own place now, he is still friends with J and plays videogames with him online. when my mother and sisters (I have 2 others) came to visit for supper, my brother told me that he was going to let K talk to J on his phone. Though she is legally old enough now to see a man his age, she was still doing it behind our Mom's back. I didn't want a repeat of what happened before and chance her getting kicked out. (though I would take her in if that happened, my brother and I are currently unemployed and can not afford to house her. So I reconnected with J online and told him if he wanted to continue talking to my sister, he would have to come forward to our mother, with a letter no shorter than 20 pages, I have been working with him on constructing it so that he can explain everything, apologize, and seek a second chance. I hope this works. To clarify, I as an older sister am very protective of K and want to make sure that above all else she safe but given a chance that I didn't get without causing division, so am I the Asshole?