Sis is pretty independent and a serious student so I didn't think anything of it when I didn't hear from her during the week. When Friday came I was surprised to find on my answering machine a call from Amy saying she was wanting to study a little chemistry later that evening and for me to wait for her to call unless I already had plans. I knew she was taking Organic Chemistry so I was not exactly sure what she was actually up to but suspected it wasn't that sort of chemistry.
I did not have a date but did have a woman grad student named Karen who was a regular I had talked to earlier in the day about maybe getting together. She was a single mother and had a young child at home and was not one of those to leave her child in just anyone's care so she often was stranded at home on weekends. She and I enjoyed each other's company and she was a very hot sex partner so we had developed that today would be a friends with benefits relationship. I called her and explained my situation and that I would be over as soon as I talked to my sister.
When Amy called she made it clear she wasn't struggling with her class and wanted to make sure I was available for later that night. She told me that she was going out with some other girls from the dorm and planned to be on her best behavior while she found out what these girls were into. I told her what I was planning on but I would be available after the bars closed. Amy asked me about Karen and her situation and I gave her the Cliff Note's version.
She giggled in her best little flirty cheerleader voice, "I know for a fact you talk and moan too loud to have sex in an apartment without her daughter hearing you!"
"It does take some of the fun out of especially since when Karen and I are alone she is louder than you," I replied.
"Sometime I will babysit and you two can go to your house and really enjoy yourselves like you and I did," she said.
I noticed a certain sultriness was coming into her voice but even after last weekend I was surprised with what followed.
"I have been masturbating all week thinking about the sex last week with you. It felt so good and it was so intense. I could let myself completely go with you without worrying about anything. Just hearing you tell me I was going to be your slut almost made me cum," she panted.
"I don't know exactly how to describe it but I feel so protected with my big brother's arms around me and it just makes me want to do whatever you want so I can keep enjoying that feeling."
"I've been thinking about this and I want you to know I want you and I to keep our personal time no matter what. I don't want to be exclusive to you or you to me. Nothing changes in our lives except we have a special relationship that we can always enjoy."
"But I won't ever break my promise that I would be your little slut after you marked me," she said.
Her voice was almost a soft moan when she said, "After all who would break a promise to family?"
I was aroused by the sexiness in her voice and I said, "I have things planned for you that are going to make you think the last time was just foreplay. Now go and enjoy the night but remember at the end of the night where you are going to be."
I actually was not sure how I was going to top last time but Amy's voice and the memory of her body were stimulating at least my cock which was still a bit hard by the time I had made the short drive to Karen's apartment. Karen met me at the door with only her nightgown on and hurried me inside before kissing me passionately.
"Kristen played hard all day with a friend and is sound asleep. If you want a beer I can get you one but I would rather we go directly to my bed room."
When we got to her bed room she dropped off her gown and was literally tearing off my clothes as we fell to the bed. I hardly did anything but provide a hard cock and some soft verbal encouragement. Karen sucked my cock until it was throbbing, then straddled me and rode it until she had to collapse on top of me and grab the big pillows she kept bedside to smother the sounds of her orgasm.
She slid off me and to my side and said, "I just needed to get off with a real cock so forgive me for acting like a teenager. I have been putting in long hours on my thesis and needed that relief."
I could tell by the sound of her voice that physically she was done for the night but I chuckled to myself thinking that I was going to be with a real teenager later. She had cum so quickly that I still had a throbbing hard on that needed relieved which she offered to do. But I told her to lay back and I ran my hands over her entire body gently. I could tell by how quickly her eyes closed and how she moaned just enough to let me know she was awake that she was getting what she needed.
As I ran my hands I talked to her about Amy babysitting and she thought it might work but would have to meet Amy first. When Amy called me telling me her new friends were getting ready to head back to the dorm I asked Karen if they could drop Amy off at her place and then Amy could ride back with me. That way she could meet Amy and see what she thought about her as a babysitter. When Amy arrived she was in the very articulate and refined young woman mode I had seen in public for years and showing no signs of the hot little slut I saw the week before. Karen was impressed and they seemed to take a liking to each other immediately. Karen told her she would have to see how her daughter Kristen reacted to her but she thought it was going to work just fine for Amy to babysit in the future.
As we left Amy turned to Karen and said, "I see why my brother likes you so much. You are very beautiful and I so enjoyed your company."
It was interesting as I watched my college freshman sister charm a late 20's graduate student so easily. I remarked about that when we got in the car.
Amy replied, "I know you said you liked the sex and her company so wanted to make sure I helped you out like a good sister should."
Then she turned to me and said, "You did save some thing for your little slut sister?"
I was still turned on by my earlier sex play and between my throbbing hard on and my knowledge of how much she liked to be told what to do I had some ideas where we were going to do.
"I saved my cum for you my little slut sister but you are going to have to earn it," I told her. "When we get back to my house you will do exactly what I tell you to do until after you have earned my cum!"
She blurted, "Hurry and get me to your house!"
Amy moved closer to me in the car and ran her hand up to the bulge in my pants.
She leaned to me and said softly in my ear, "I want this in me again and cum running out of my pussy."
I said with no real enthusiasm, "We need to keep a certain appearance in public but when we close the doors we can let our inhibitions go."
Amy moved hesitatingly away although her hand lingered and she replied, "I totally agree we should be careful but no one can see where my hand is outside the car."
