My sister walked in.

"Um, what are you doing?" My sister asked me, obviously a bit intrigued as to why I'm holding this doll.
"Oh, hey Steph. I was cleaning my closet, and I found this doll in my old box of toys. I don't remember having this though. Do you?"
"Here, let me see."
My sister knelt down beside me and took the doll. As she studied it closely, for a moment I couldn't help but catch a glance at her perfect breasts. They looked so enticing in her tank top. She was 2 years younger than me, and for some reason I found her to be so friggin' hot. I shouldn't think that way about my sister though...
"Yeah, I don't know John, doesn't look familiar." My sister put the doll down onto the floor and stood up again. "Anyway, I came in here to ask if you knew where the band-aids were. I have a small cut on my ankle, and I just wanted to patch it up. It doesn't really hurt, it's just irritating."
"Oh yeah, I think I saw those under the sink in the bathroom." She hasn't gone out at all today, how the hell did she cut her ankle? What a klutz. A really sexy klutz...
"Okay, thanks," Steph began to walk out of my room when I looked back down at the mystery doll. There was a small red stain on it. Steph's cut must have leaked a drop of blood onto the doll. At this point, I might as well throw away this unknown and now stained doll. As I picked it up, something strange happened... It changed! It now looks exactly like my sister!

What is this thing? I studied closely, and was amazed at how detailed it was... It matched her exact outfit, and her hair was the perfect length proportional to the doll's size. Her breast size was even perfect! Then I saw the little red dot on her ankle, matching the real cut she had. This was amazing.

I quickly left my room with the doll to show my sister. As I neared the bathroom, I came to an immediate stop. The bathroom door was open, and my sister was bent over trying to find the band-aids. She hadn't noticed me, and her ass was sticking out for my viewing pleasure. I couldn't look away. I got so distracted I almost dropped the doll. As a reflex, I tightened my grip.
"Ow!" My sister yelped just as I tightened my grip. I panicked and headed back to my room before she saw me.

My heart was pounding. I'd discovered something amazing. This doll affected her. I had to try something else, and see how far this could go.