My dick accidentally popped out in front of my wife's little sister

I have been with my wife since we were kids. She has 2 younger sisters and we have always been close. A while back we were living in an apartment and we went through a period where her youngest sister, who still lived at home was spending a lot of time at our place. She was like 19 at the time, very cute party girl type who loved to drink. She wasn't getting along with her parents so she crashed with us a lot.

My wife was working nights and I was in between freelance gigs, so me and the sister spent a lot of time together. Not that I was complaining, about the cute young redhead walking around in a towel and hanging her underwear to dry in my bathroom.

She slept on our couch, and wasn't exactly an early riser. Well one night she mentioned they switched her shift and she had to get up really early for work the next day. I told her I would get her up. My alarm went off at 4. My wife was working overnight so she still wasn't home. I sleep in my underwear, had she been home I probably would have put on pants to go wake the sister, but it was the middle of the night so I just went in my boxers.

I told the sister it was time to get up, and she didn't budge. Not exactly surprising. I noticed her blanket was all pulled to the side and she was sleeping in her underwear too. She was wearing a tight little pair white cotton panties that hugged her mound beautifuly with a little pink bow on the waistband just below a small tuft of red pubes poking out. I definitely looked a little longer than I should have, and wound up with a decent halfstack. I realized I should stop staring and get her up. I leaned over, gently shook her shoulder, and again told her to get open. She opened her eyes and mumble something. I couldn't hear so leaned in closer and said "what?"

Very calmly she said "Your dick's out" I was tired so it took a second to register and before I fully understood what was happening, she grabbed my semi with 2 fingers. I had forgotten to button my fly and it was just hanging there pretty much right in her face. So she calmly picked it up with those 2 fingers and again said, "dude... Your dick fell out. As she held it it obviously started to get bigger. She gave a little laugh and put it down. "You should probably put it away now." "Oh shit I'm so sorry I had no idea" "Oh relax I've seen a boner before." "Thanks for getting me up. I'm gonna go get a shower while you take care of that"