it started with you grinding your ass on your big sis cock at the cinema and now you are both fucking with each other everywhere while trying to expose the other.
at first it escalated bit by bit, handjobs in public bathrooms when they were empty, then blowjobs with the public bathroom with just a few people, then fucking with the public bathroom full.
you think maybe the point of no return was when your big sis was grinding her ass on you at the park and you pulled her pants to fuck her properly, trying to make HER cum instead of you, when someone heard the noise and approached she was trying to get off of you and get dressed, but you kept going, and you both came at the same time in the face of the stranger.
That was a draw of course...
from then on, each time you went out with your sister the "hang out" went as follows:
You both get out normally, like sisters/girlfriends.
The teasing gets one of you hard.
You both look for an uncrowded place.
Public demonstrations of "affection" begin.
The affection becomes loud and each of you try to make the other cum before youselves.
The noise attracts some people. Sometimes the uncrowded place becomes very crowded very quickly.
One of you comes in front of the crowd, followed just seconds from the other.
Depending how much people see the person cumming it may become a record, and then the "Most Watched Bitch" title may change owner.
"Most Watched Bitch" has to do the chores until the title changes again.
Now the record is 6 people and you have to do the dishes and clean you sister clothes. She comes to hand you dirty panties from time to time, interrupting you of whatever you are doing, sometimes she hands you a pair while you are still cleaning, one time she handed you your own sock filled with her cum.
You need to think of something quickly. But where or when should you attack?