When I went to my friend's house to play, my friend's sister entertained me, but I couldn't find my way around her super big breasts. Then, by chance, I encountered my sister completely naked while she was changing in her room! When I saw her super big breasts, I got an instant erection as an adolescent! Even if I tried to hide it, I was found out immediately! I thought she would find it disgusting, but she was also in heat and invited me with a grin! I secretly and gently had sex with her without telling my friend!