Making me cum on in the tightly packed family car

This is about the trip I took back in Highschool with my then girlfriend and her family to Yellowstone. We had just spent 4 days at the campground and began our drive back home which was about 12 hours away. It was a hot sunny day and we were all crammed into this small van. There were six of us in this little van, her mom and dad in the front, one really young brother and the two sister, 13 and 15, whom were in the middle and then it was my girlfriend and I in the very back. The Van was stuffed full, there were tents, suitcases, bags, and food stuffed in every spot available. You could barely see us in the back under the mountain of gear. My girlfriend at the time was this cute brunette, she was bout 5’3” and was very tiny. She was on the track team and had a runners look through and through. She had small breasts and a tiny ass that was tight a perky. We had spent the week together without any privacy and it was starting to get to us. Me being 17 I was constantly trying to hide my erection that popped up every few minutes, and being around my sexy girlfriend made it even worse. We were about 2 hours into our trip home when my girlfriend asked to lay on my lap so she could take a nap.  As soon as she laid her head on my lap I could not ignore the pressure of her head on my crotch. After a few moments my cock was fully hard and tenting my pants under her head. She looked up at me with a devilish grin and started to rub my cock  through my pants. I started to subconsciously hump  up against her with each stroke.   She turns around  so that now she is now facing me with her back to the front of the car, and quietly begins undoing my shorts. As she slides my shorts down my cock springs out and is now inches from her face as she lays on my legs, slowly stroking my cock. I am trying my hardest to act natural as my girlfriend strokes my now exposed cock openly in the back seat of her family's van. Percum begins to bead and drip from the tip of my cock and without any warning she sticks her tongue out and licks it up. My hips involuntarily buck forward as I feel her soft wet tongue slide under the head of my cock. She can tell she is driving me crazy, and this only gives her more motivation to torture me. She licks her hand, getting it slick with her spit, and resumes slowly stroking my cock. You can barely make out a rythmic “shlick, shlick, shick” as the radio plays over the speakers. I nervously look around the van feeling  myself getting closer and closer. My girlfriends picks up my cock and sucks one of my balls into her mouth, swirling her tongue all over before turning her attention to my other one. Knowing I won't last much longer, I look down giving her a panic looked as I cannot stop the inevitable. My balls tighten and slide out of her mouth as the first huge rope of cum erupts from my cock. If flies through the air and lands right in her 15 year old sisters long dark hair. My girlfriend quickly attempts to get my cock in her mouth,  but not before another rope sprays across her cheek and into her hair. She slams her mouth down my cock as I spray the rest of my cum down the back of her throat. I have to use all my strength not to moan as I grip the back of my girlfriend's head pushing my cock deeper in her throat. As my orgasim subsides I slowly come back to the reality of what I just did in the back of her family's van. My girlfriend give me a last few licks before allowing my cum covered cock to fall from her mouth.  I slowly slide my pants back up and wipe my girlfriend's cum covered cheek. The back of the car smelled like cum for the remainder of the trip and I am not sure if her sister ever realised there was a big glob of cum in her hair. Though at the end of the trip her sister did  give me a bratty look while chewing on her hair.