Katie was 14 and I was 12. We had just moved back with our mother and were staying with her and her current boyfriend Mark. They had a one bedroom apartment together and were not quite prepared to host my sister and I at the time we arrived to live with them. Let me start by saying that my sister and I have always been very close. The products of a divorced household at an early age; we were forced to grow up quickly. We had always spent a lot of time alone together, our parents were always a little too involved with their own lives to pay much attention to us, and we had always been fairly self sufficient. Katie was, in essence, my live in babysitter. She sort of surrogate mothered me all of my life and also stood as my closest friend through thick and thin. Although neither of our parents ever really sexually abused us, we were always very aware of what sex was and the fact that our family wasn't very shy about their own sexuality. My father would occasionally walk around nude on the way to the shower, a huge erection leading his way. Our mother was known to moan rather loudly on occasion. Regardless, it never seemed dirty or anything, and I believe our parents never thought of us in an overtly sexual manner. Like all other siblings, my sister and I were accustomed to the occasional tickle fight. These "fights" always started out innocent enough, but usually ended in my sister being angry (she really hated to be tickled). So one evening my sister and I were alone and I was feeling ornery, so I started tickling my sister as she lay on the couch watching TV. Katie started squirming and giggling, which always threw me off when she would get so mad. She was in her usual pink shear nightgown. It was worn so you could almost see her developing tits through the flimsy material. As I tickled her I felt myself becoming aroused. I was in just a t shirt and underwear, as it was a very hot summer night. I jumped on top of Katie to get a better tickling position, and pinned her arms down with my knees. As I continued to torment her she began to get angry. I let up a bit and asked her, "If it bothers you, why are you laughing!" She looked up at me and said, "You're smashing my fucking tit!" I looked down and moved my knee off of her tit. The nylon of her nightgown was stretched taught against her breasts. Her tits were a foot away from me displaying themselves openly. They looked so plump and big with the way they were bursting through her nightgown. The nipples were erect and therefore all the more appealing.