My Brother's Birthday Gift

Yesterday was my brother’s sweet sixteen birthday. As told earlier, my sister and my brother are very close. My sister promised that she will give my brother a wonderful g*ft but he declined and said that he wants something from her. He said it will not cost any money rather my sister just have to do something for him. As my sister loves my brother more than me, she said yes. I did not know what it was until at night.
It was around 12:30 am when I heard sounds from their room. My room is adjacent to their room so sound passes easily. They don’t usually close the joining door and I could see lights coming out under it. So, I peeked into their room from the joining door. I saw that my sister was lying on the bed naked with her legs apart. My brother was standing near her touching her all over her body. I first thought that she was asleep but I saw her talk to him and guide him around her body.
From where I as standing, I could see my sister’s slight hairy pussy from front. My brother was rubbing his finger over her pussy and then slowly towards her boobs. As told earlier, my sister has good size boobs (bra 34C). I could also see that my brother already had his tent in his pant. As far I know, he saw her naked first time in more than 10 years. My sister seemed to enjoy his touch. Then she sat on the kneeling. This time I could see her butt cheeks. They were slightly proportionated to her boobs. He started to press them. By this time, I realized I also got a hard on. Though I never feel sexually attracted to my sister but biologically, seeing a naked girl aroused me nonetheless.
Obviously, my brother could no longer keep his things in his pants and let the pants drop on the floor. It seemed to me that my brother has grown a bit than the last time I saw. My sister took it in her hand and stroked it a few times. My sisters back turned to me and I felt that my brother’s face was in her boobs. Possibly he was sucking them. Then he came around and slapped her butt a few times. Then he suddenly said that he feels tired. So, they turned off the light and went to sleep.