Never thought I'd tell anybody about this, but ...
My sister is three years older than me. When I was in college, she already had a job as an office assistant and had her own apartment. I still lived at home; she gave me a key to her place so I could escape if I wanted. We had a very good relationship, with open discussion and no secrets (so I thought!). Her roommate was a brown lab that she adopted from a shelter.
Most weekends I would go over to visit her, but on weekdays I was busy with school and friends, and she was busy with work, so I rarely dropped in. One night during finals I was pulling an allnighter but having trouble staying awake. Around 2am I decided to make the trek to my sister's place for a change of scenery.
I assumed she was sleeping, so I snuck in quietly. I could hear the television on in her bedroom as I settled in at the kitchen table. Then I heard her making little noises. They didn't sound like sex noises; my first reaction was that she was having a weird dream or something. but they continued for minutes and minutes, so I went quietly down the hall to see if everything was cool.
The light from her television lit the room dimly and her door was about half open. When I peeked around the door jamb, this is not what I was expecting. She was fully naked, on top of the bed, flat on her back, fat ass wriggling, knees in the air, legs wide open, large breasts heaving, and the dog's head between her legs, licking away. The little chirping noises were like squeaky gasps. My sister is a bit overweight (hence the fat ass comment), but she has a nice voluptuous shape to her body and really big tits. I'd seen her naked many times before (getting dressed after a shower, that sort of thing), but never saw her do anything sexual before; I was stunned.
I quickly ducked away from the door and debated what to do next. I'd never seen anything like that and had no clue that my sister(!) would ever consider such a thing. As I tried to decide what to do, her noises were coming faster and faster, and a little louder. I had to look again, there was no way I wasn't going to get an eyeful of this, it was just too crazy.
I slowly peeked around the door again, holding my breath. There was little chance she would see me, unless she suddenly sat up, so I watched for maybe 15 or 20 seconds. She was really into it. Scooting her ass all over the place, squeezing her tits and nipples. I could hear the dog licking and it sounded like a sopping wet mess in her pussy. It was amazing, and my cock started to move in my jeans. She continued to increase the rate of her noises and her orgasm seemed imminent, so I ducked away again and listened. About 30 seconds later she was done. She yipped and chirped and shifted and bounced on the bed and the springs squeaked.
At this point I thought she might come out of the room so I scrambled back to the kitchen, thinking I could play it off as if I didn't hear anything unusual. After 5 or 10 minutes of pretending to study, she didn't come out, and I didn't hear any more noises. I decided I would sneak out and go back home, but I wanted to risk one more look. (Also, part of me wanted her to know that I was there.)
When I looked in, it was even more shocking than the first time. still naked, sitting on the bed with her back to me, the dog was standing on the bed next to her with a rock hard dog cock and my sister was jacking it off. I watched her boobs swaying quickly to the rythm of her hand movement. While my sister couldn't see me, the dog could, and he turned his head right at me and stared. This caught my sister's attention and I could see her hand start to pause as her shoulders started to turn towards me.
I ducked behind the door and held my breath again. I tip-toed down the hall, and I thought I heard her get up off the bed, but it was hard to tell. Again, she didn't come out. After a few minutes, I went back to her room again. When I peeked in, she was on the bed still, on all fours, her big fleshy ass up in the air, her huge tits hanging down. The dog sitting on the bed enjoying a blowjob from my sister.
His dog cock was huge. My sister was using both her hands on the shaft while working about 6 inches in and out of her mouth. I barely had time to register the scene when the cock popped out of her mouth and she said, "Hey, I know you're there!"
She told me to just come in already since I'd discovered her secret. She was acting a little shy, but also realized the cat was out of the bag. "Now that you've caught me, I want to see you hard. Pull it out and play with it while I do this", she said. I was hesitant, but too turned on not to comply. I dropped my pants while she went back to work on that big ugly dog cock.
She was making a mess of it. I don't know if it was her saliva, or the dog's pre-cum, or what, but it was a juicy glistening mess and it looked amazing. She looked like a total slut sucking off that sausage, waving that big round ass in the air, her heavy tits swaying, nipples touching down to the bed. Her pussy looked very wet still and her mouth and hands kept moving all over that doggy dick. Before I knew what was happening, I was shooting cum all over the place. She paused for a few seconds to watch as I shot load after load onto the floor, onto the bed, everywhere; it felt uncontrollable, like I was going to coat the whole room in cum.
I sat on the bed to catch my breath and watch the rest of the show. The dog was surprisingly nonchalant about the whole thing, I had the feeling he was an old pro at this. After a few more minutes, the dog started cumming all over my sister's mouth, hands, face, and bed. She was a sloppy mess. Dog and human cum everywhere, her crotch was dripping wet, her bed looked like a hurricane came through.
The room smelled like filth; sweat, dog, cum and wet pussy combined. I'll never forget that smell.
We didn't say much to each other afterwards. It wasn't awkward, there just wasn't much to say. We cleaned up as best we could, and I tried to turn my thoughts back to studying, but it was impossible. After this, we had a few other experiences with each other, but this one will always stand out as the hottest in my mind.