My sister and I get along very well. We shared a bedroom until we were both in high school and were very open about our bodies when we were young. When she lost her virginity in college, I was the one she came to and when I lost mine, she was the one I told.
When I was about 12 and my sister was 13 or 14, We decided to see how big my dick was. I laid down on my bed and pulled my clothes off while she went to go get a ruler or measuring tape to measure me. We started by measuring me while I was still flacid and my measurements were about 2.5 inches long and 1 inch across my glans. I am circumcised just for the record.
Then we decided to measure me with a hard on. I rolled my member around until I got an erection and my sister started to measure me again. She had a little trouble because I didn't get as hard as I do before I ejaculate but when it stopped growing, it was about 6.5 inches long and 2 inches across.
My sister then wanted to see me actually ejaculate so she sat down with her shirt open while I rubbed my shaft and head. Unfortunately, I couldn't reach orgasm. I asked my sister to suck it but she said that she wouldn't but she would be willing to rub it herself. Not being one to turn down a good offer I said that she could so she stripped down to her bra and panties and started to rub me.
After a while, even though I was hard enough that it hurt, she laid down on top of me and wrapped her arms around my back. Her panties were right on top of my hardness. I started to get into it and began sliding my dick on her clothed privates until I felt my orgasm building. I told her that I was about to cum but she said that she wanted to feel me under her when I shot my load.
When I came, she had a little orgasm with me and I shot about 7 times on her mound and belly. We laid there for a bit and then got up and cleaned ourselves off before our parents caught us.
Later at school, My sister told her friends how long I was both erect and flacid but she didn't tell them the rest.