My kid brother knew I had tentacle anime lesbian porn on our PC (and I am a female) when we were both teenagers. Heck, he even came home once when I was having fun with my ex-bf and he never said anything to me or my then-bf even when he didn't obviously like the current bf. I didn't tell anything about him to parents, and he did the same favor for me. He got caught smoking (just cigarettes, but my mom hates smoking) when he was 18 and I was 20 and not living at home anymore, but at that point he ratted out to mum "but Suvi smokes too!" Mom calls me, I tell her that I did start when I was 15 and basically quit when I was 17, then smoked irregularly and started picking it up again at 19 (and now I've quit totally). I think my bro thought that since I am considered an adult by my parents now and the Good Kid anyways, the deal is off and I can defend myself if I have to argue with my mom. Which he was right about, I had wanted to talk about it and ask mom didn't she REALLY ever suspect anything. So, make your bro an ally if he seems trustworthy. If not, do you know something he wants not chat about with mom and dad?
