There were two sofas situated facing each other in the living room of the apartment we had at the time, and when we got home my brother sat on one while I put some low music and dimmed the lights and Natasha went to fetch my good whisky. I made sure I was done before her and then lay on the other sofa in the hope that Natasha would sit next to Mark. However, my heart sank when she came in and sat with her knees bent on the floor between us. Had I just been imagining her attraction to him?

Twenty minutes had passed and, to my horror, the conversation had become a little mundane. It seemed that my presence in the room was not exactly an aphrodisiac. I thought about going to bed and leaving them alone together, but that would mean I would be stuck in another room and missing out on everything. I decided on another course of action and began to fake drifting off to sleep – breathing heavier and making slight noises. My eyes were closed, but I could just see through the gaps every now and then.

The conversation carried on down its banal path for what seemed like another half hour and I was just about to give up on my plan and go to bed when Natasha called my name out softly to see if I would respond. I continued to breathe heavily and then she repeated it: “Thomas?” 

There was a silence for a moment and then my brother tried: “Thomas, don’t you want your drink?”

I made a grunting sound and continued the slow, heavy breathing. They remained silent for a moment, and my realising that they were checking to see if I had fallen asleep sent my cock throbbing harder and harder.

Though the conversation over the next few minutes remained almost as innocent as it had been before, their voices lowered and their tones gradually became softer and more intimate. Through my cracked eyelids and the dim light, I could see them look over every now and then to check if I had woken.

Gradually, as the whisky started to take effect, their conversation became less and less elementary and they began to talk of more personal things. By this point, I was already so turned on I could hear my heart beating in my chest and I thought it might explode.

I knew that if they caught me peeking, the game would be up and that I would have to forego the opportunity to occasionally glimpse at them if things were to progress. So, I decided it would help matters if I were facing away from them and made a few grumbling noises as I turned over.

Within a couple of minutes, it seemed to have worked. They began to talk about my brother’s failed marriage, and he said that he thought they stopped finding each other attractive. Then, in a softer voice, he said, “Thomas is so lucky to have you.”

“What do you mean?” I could detect the hint of anticipation in her voice.

“You know exactly what I mean.”

There was silence for a moment, and I could feel her peer over to double-check I hadn’t woken.

“You shouldn’t say things like that,” she protested feebly.

“I know you know I have fancied you since we met this morning. I know you feel the same way.”

“You shouldn’t say that.”

“Why not? It’s true, isn’t it?”

“You know I can’t answer that.” The sensual tone in her voice almost invited him to keep pressuring her.

“Why not?”

“Because…”

I could feel her look over at me.

“You’re as turned on as I am, aren’t you?” he pressed.

“Yes,” she whispered seductively.

My cock was dripping and heart was pounding so much I was sure they could hear it. Then, I was suddenly startled by a hand shaking me on my shoulder, and I heard my brother saying I should get myself to bed. I had never been so turned on and was desperate to continue spying on them, but there was nothing I could do but pretend to have just been awoken.

I groggily asked what time it was and Natasha responded, “It’s past twelve and you were up early, let’s get you in bed so you can have a proper sleep.”

I took her hand and she led me to our bedroom. Careful not to let her see my erection, I stripped off to my underpants and got into bed and closed my eyes. She leaned over and kissed me on the cheek and said she’d be in in a bit, before switching the lamp off and quietly closing the door.

I didn’t know what to do. I couldn’t believe I had been shut out and there was nothing I could do about it. I suddenly felt extremely jealous, but that soon subsided at the thought of them together.

I lay there in agony and ecstasy, imagining him running his hands over her satin-covered breasts and teasing her hard nipples as she stroked his hard cock.

I had to sneak down the hall to spy on them but knew it would ruin our relationships if I got caught.

I knew they’d be concerned I might get up and discover them, so I needed to give them time to feel more assured that I was fast asleep.

After waiting for what seemed like an eternity but what the alarm clock showed was twenty-five minutes, I thought it might be safe to make my move.

I crept out of bed and slowly turned the handle on the door with trembling hands before pulling it open inch by inch to make sure it didn’t make a sound. My cock was dripping as I tip-toed down the hallway towards the living room. As I got closer I could hear the faint sound of their voices, but couldn’t hear any actual words. When I got to the door, I realised that what I could hear was Natasha moaning with pleasure.

I stood there for a few minutes listening to her pleasure and knew I needed to see what they were doing. There was a glass window in the door, but I knew that they were to the left of the door and that if I were to peek through and they were facing my direction, there was every chance they would see me. Although I knew I would be risking everything, I had to look. 

I took a quick peek through the glass and could make out Natasha on the floor. To my relief, she was facing the other side of the room and could not see me. I peered through the glass again, this time more slowly, and could see her on her hands and knees; his hands were on her breasts and her body seemed to contort with pleasure as he squeezed her nipples and fucked her harder and harder.

“Oh god yes, fuck me,” she begged as she felt each thrust of my brother’s hard cock fill her desperate, cheating pussy.

My aching cock could take no more and I felt my pants fill with burst after burst of warm cum and I suddenly realised that it was a matter of time before they’d see me, and I quietly made my way back to bed. I waited there for another hour and forty minutes before I heard Natasha slowly open the door, undress and slip into bed with her back to me. The aroma was fantastic, a heady mix of sex and cum.

Before I knew it, I was awoken by the sound of Natasha opening a drawer. It was Saturday morning and Natasha was finishing getting ready for the gym. She grabbed her bag, kissed me and said she’d be back by twelve.

I heard the front door close and then waited a few minutes to give her a chance to come back for anything she might have forgotten, before looking on the floor for the clothes she had been wearing the night before. Unusually, they had not been left there, so, already throbbing with excitement, I went into the bathroom to check the laundry basket.

My hands were trembling with excitement as I lifted her silky satin blouse. I still smelled of her perfume and, as I inspected it, I saw dried cum marks up the front and on the collar. I pulled out her panties and discovered they were still moist. The unmistakable smell of pussy and cum was still so intense and I lifted them to my face and breathed it in deeply before tasting it. Then, unable to resist any longer, I took her blouse and gently rubbed the silky material on my eager balls before wrapping it around my aching cock and covering it with streams of hot, creamy cum, as my brother had done while Natasha was wearing it.

I guess they felt guilty about what had happened, and after receiving a few text messages in the morning (probably from Natasha), my brother said his ex had booked him a room in a hotel near hers.

As I alluded to in the title of this account, for fear of it harming our relationship and harming the family, I have never even hinted that I have any idea of what they did that night. Believe me, I have been, and still am, desperate to tell Natasha how turned on I was by it, and that she needn’t feel ashamed, but I know it would probably do more harm than good. Besides, that night I learnt an important lesson: the act of cheating is part of the turn-on for Natasha.