Sister's Underwear

My name is Mark and I'm a young male who has suffered through hormones like any guy. But is there anything wrong with me for being attracted to my older sister? Growing up similar in age, we "fought" for use of the shower among other things. When I hit puberty, my mind randomly went to my sister in a bra and panties. No reason, just Emily in her undies. Just typing that gives me a raging hard-on. One day I saw it for real: Emily was wearing a lacy pink 32DD push-up bra and white cotton panties. I never knew her boobs were that big! Here Emily was, brushing her hair in front of her mirror while rubbing that pretty bra of hers and humming some pop song. Then I saw something that I'll never forget: Emily stuck her hand down her underwear and started to masturbate. Being a young male, this was amazing. My penis was straining against my own briefs at this point. Emily was swaying and writhing away, and started moaning. My dick was inflating at such a rapid rate, it was all I could do to avoid being caught. Then the magic happened. Emily bent forward and screamed in ecstasy. As she was bending forward, I noticed those white undies were getting wetter and wetter. Emily was too busy catching her breath and lifting those wonderful tits in that bra to notice me. Or so I thought. Soon after Emily orgasmed, I did too. Screaming like she did, my briefs were being hit by my orgasm too. Emily couldn't have missed it. My white briefs were rapidly becoming darker. Fortunately Emily was cool with my being turned on by her. So she masturbated once again. Right in front of me! Lets just say that both of our underwear permanently changed color that day