Both characters in this story are over 18.
That day was hot. Really hot. We had AC, but I was forced to run some errands, and came in with my clothes clinging to me like I'd jumped into a pool. My sister was at work that day, so I decided to just take everything off and sit in the AC in my underwear, and maybe watch a movie.
Ten minutes later, I hear from behind me:
"Good fucking idea." It was my sister. She started to strip down to her underwear, while explaining that the power had gone out at work, so they sent everyone home. Something I should explain about my sister. She's very...voluptuous. I don't really notice most of the time, but now she was sitting there, a few feet from me, in her skivvies, it became very hard not to. My sister has very large breasts, like F-Cup range, and an ass so big, only a real man could appreciate it. Also, she's a bit soft around the mid-section, but I like that in a girl.
So, my sister was watching the movie I put on, but I was too distracted, looking at her tits in that black bra she had on. I know, I shouldn't look, but I couldn't help it.
Suddenly, I felt my cock twitch, and I started to panic. Not only was this severely fucked up, but all I had on was a pair of tighty-whiteys. It was gonna be hard for her not to notice I've got a 2x4 in my jockeys. Yeah, I have a big cock, I just prefer not to make a big deal out of it. I tried hiding it, but she noticed my efforts to do so.
"What are you doing?"
"Nothing." It took her all of a second to figure out what I was doing.
"Do you have a hard-on?"