Cheating with my girlfriend's sister

I've been dating Karen for 2 years now. She has a HOT younger sister named Emily that just turned 18. She's blonde, has a slim but curvy body, an AMAZING ass, and boobs to die for. I was really tempted for a while but I love Karen. I was going to ask her parents to marry her. I went over, unannounced and her parents weren't home. But Emily was. Emily invited me inside and told me they'd be home in a little bit. I sat down on the couch, across from Emily. She also sat down and we talked. She was wearing tight blue jeans with a very booby shirt that I was trying to not stare at.
She said "Sooo how are you?"
"I'm good, really good. I was actually coming to ask your parents if I can marry Karen"
"No fucking way! Are you serious! That's awesome"
"Thanks. What's new with you?"
"Nothing much. I'm just finishing up school"
"Nice"
She said" I know, right? I can't wait" as she sexily sighed.
Oh my
She gets up and walks towards the minifridge on the ground, bends down and opens it as she's practically showing off her ass.
"Do you want anything? We've got some cold sodas. They've been in ice for a while so they're kinda wet" she said as she shakes her ass.
I was focusing on her amazing ass and I barely noticed she was looking back at me for an answer.
" were you looking at my ass?"
Startled, I looked up at her and said " no"
"It was obvious"
" I'm not attracted to you"
She sexily walked over and said "then I guess you won't mind if I sit on your lap"
Then she sat on my lap, no, scratch that. On my crotch.
I was trying so hard to not get a boner and she opened her soda and took a sip and sighed while she moved her hair around.
Oh my god
I was trying so hard to not get a boner but it was too late. She felt it and looked oddly impressed by the size.
She then rocked a little bit on it and looked back at me and bit her lip.
That's it.
"Lets do it" I said as she got up and took my hand upstairs.
We went into her room and shut the door. I was quickly removing my clothes as she did the same. I kissed her, felt her tongue as she removed her bra and I got to see those big, natural tits. I laid down as she took off my pants and boxers, which revealed my cock. She smiled and licked the tip. She looked back at me and slowly sucked on the tip. I grabbed her head and bobbed it up and down. She opened her mouth and slowly sucked the whole cock. I sighed in enjoyment. After a few minutes, I told her to get up remove her panties. She did get up and bent her ass in my direction and slowly removed her panties, revealing her tender pussy.
She laid down and i smacked my cock on her ass. My pre cum covered her butt. I slowly entered and thrusted.
"Fuck me faster baby"
I did and boy, her pussy is so much better than Karen's. It was so tender I came pretty quickly.
We both laid on the bed, amazed.
I'm fucking her from now on.