Great story, and good luck in December! If it were a porn movie then they'd both come up to you in December and say that they wanted to try you together
Unfortunately for my my gf's sister is 20 years our senior, and hates my guts. I'd like to pretend that she's just hiding underlying sexual tension under a mask of loathing, but chances are she really hates me.
The closest thing that I've had to a story like yours was when I was about 20. My gf was out of town, and she asked me to look after her best friend who was living by herself while she was studying. My gf was quite explicit that I shouldn't let her friend go home alone, as she thought it was dangerous.
Every now and then I would stay overnight at her friend's house, because there was no public transport that late at night near her place. Nothing ever happened. Still, we developed a friendship during that time, and she went from being my gf's friend to being my good friend.
One night I stayed at her place. We chatted until late and we both fell asleep, fully clothed, on her bed. When I woke up it was early morning, and I had my arm around her, my hand holding on to one of her ample D cups, and a raging hard-on pressing against her ass from behind.
OMG I thought to myself. I wanted to keep my hand there, but I was scared that when she woke up she'd go crazy. I slowly and carefully removed my hand, and pretended to sleep. She then got up, and I was thinking "was she just awake?". She took off her clothes, and changed into her night dress, as I peeked through squinted eyes. I didn't see her body clearly, but my imagination was running wild and my hard-on was harder than ever.
She got back into bed infront of me and she ... wait for it... SHE put my hand back on her breast!!!!
Her now un-bra'ed breast!
I left it there for a while, sometimes moving, sometimes squeezing, all whilst trying to pretend to be asleep so as not to feel too guilty. Lil bro was shouting "do it!"... and I really wanted too... but the guilt took over. She, after all, was my gf's best friend!
I pretended to wake up, and she pretended to sleep. I went straight into the bathroom and started to HJ away my huge load. Suddenly there was a knock at the door: "What are you doing in there?"
"Nothing!" I lied back to her
"Can I come in?"
"..."
"Hey... Tell me what you are doing"
I would have told her. I would have invited her in. I was ready to cheat on my gf. But then and there I exploded... and spent the next minute cleaning up.
Guilt ridden and slightly embarrassed, I made my excuses and went home. The chance never came again, but she's still on my to-do list... and just last month she was added to my Facebook friends list too.
Living in hope