You come into my room and I am changing. I see your penis is hard and make fun of you for it. You specifically are staring at my armpits. You like my armpits? Omg my armpits are turning you on! That's so funny. Let's make a deal. You jerk off in front of me right now and I won't tell anyone that you walked in on me and that you are obsessed with my armpits. You agree and while you are jerking off I am giving you lots of dirty talk and showing off other parts of my body. I show off my tight asshole and spread my pussy for you. I end up joining you in masturbating and we cum together. Hmm maybe you should come into my room more often step-brother ;)