Fcuking Younger Brother's Girlfriend

I am a final year engineering student. My younger brother is in class 11th. He has a girlfriend who is 2 years junior to him in school. They recently broke their virginity. My brother and me are close so we know each other's dirty secrets as well. 

So last week, my brother got his GF home to have sex. To give them their privacy, I was upstairs in my room and studying. Suddenly I got a call from my brother asking me to come downstairs. I thought he must have forgotten the condom so I took one and went downstairs. Both of them were in our parents' bed and naked. And the girl was blindfolded (they were probably trying BDSM). The girl was lying naked on bed and legs wide open. And my brother was standing with his dick in his hand. He was wearing a condom. 

Looking at the girl naked, I thought she might not be aware that I am here. So I quitely asked brother, 'what happened?' He said in normal voice, "I am not able to insert in her pusy"

Since my brother's tone was normal, that meant she was aware that I am gonna be there. But she didn't cover herself. That was strange for me. But then to solve his issue, I saw his dick was very well erect. Then I saw her pusy. It was not at all wet or open. That meant she wasn't horny yet. I told brother to make her horny, he asked me "How?"

To be honest, I will be lying if I say I wasn't attracted to her. Obviously, who wouldn't be attracted to a girl who's naked, lying on a bed with her legs wide open. I went to her, grabbed her closer to myself and started rubbing her pussy with my fingers. She started with her little moans. I kept going on and increased my speed slowly. She was moaning louder than earlier and her pussy started getting wetter. And obviously my dick was getting harder. Brother was standing there and enjoying her moans and learning from me. Then to increase my intensity I began fingering in, and her moans almost doubled in volume. Everyone in the room was getting hotter, and my brother began masturbating. 

Then I paused for a bit. The girl relaxed, breathing heavily. I looked at my brother and asked if he wanted to continue. He said, 'Tell me more ways to seduce her." I was waiting for the same moment. Why would I let the chance go. By this time, girl had removed her blindfold. I removed my pants and took out my hard dick. Put it over her pussy. Seeing this my brother got even more hornier and started masturbating rigorously. I was rubbing my dick over her pussy. She was enjoying it. By this time, brother had ejaculated and his cum was dripping on floor. In no time, I inserted my dick into her. She let out a loud moan. Obviously there was a lot of difference in sizes of my brother and mine. And her pussy was way too small to take it. But she did. I completely ignored my brother now, dragger her near me and began fcuking her. With my each stroke, she was getting moved on the bed. I had to drag her back to me again and again. I saw tears in her eyes, but why would I care. Her moans were telling me how much she was enjoying it. And I really don't know why my brother did not stop me. 

After a good fcuk, when I was about to ejaculate, I pulled out and the showered the cum on her navel and waist. I wanted to act like nothing happened. So I just looked at her, crawled on her, sucked her lips hard, got up and left the room with my pants. I don't know what happened in that room later, neither I asked my brother anything about it. But she followed me on Insta the same night.