My girlfriends sister_(1)

First of all this is my first story so I will let you know a little about me. I am a 19 years old male, and I go by the name Bob. I have short brown hair and blue eyes. I am about 5 foot 10 inches and weight about 175 lbs. I am as some like to say well hung, about 9 ½ inches. The other person in the story is my girlfriends little sister Beth. She is 15 years old and is not all the big. She is about 5 foot 2, and might weight 95 lbs soaping wet. She has shorter (not like a butched) red hair, and hazel eyes. For the most part she is an average small girl, small boobs (A cup), and a small waist. For a long time she acted like she hated me, but one day about a year ago she started to do some flirting with me. When I would lay on their couch she would let me rest my legs on hers and she would put her hands on my legs and in a way she would give them a little rub. She would also hang on me sometimes when we were looking for something to eat. This would not be anything weird to be except for the fact that she never wanted anything to do with me one day and the next she did all this stuff.
So it was about a week ago and my girlfriend was at work. I had borrowed her dad’s chain saw to cut some trees down in my back yard, and was bringing it back. I figured to be a nice guy I would swing in the house and tell Beth that I had brought it back, and also to see how she was doing (ok you caught me I wanted to see her to.) She was sitting on the edge of their couch watching TV, and I sat down on the other end to talk a little bit and see how things were going. She was eating her favorite food in the whole world, pickles. I always liked watching her suck all the juices off the pickles. It made me think about what she could do with my cock. I like always she and I made a little bit of small talk, until she asked me something strange. She asked me if I would ever think about cheating on Ayrn (my girlfriend?) It caught me a little off guard and I did not know what to say.