I don't have any sisters, but here is a story about being tickled by my female cousin.
I did post this before, but it is in the "Early Experiences" monster thread, and would be hard to find among the vast number of responses there.
When I was eleven years old, on a summer day, my parents took me along on a trip to my aunt and uncle's house upstate. My brother, at 15 was considered old enough to leave at home.
The four adults went to shop for cars. That was the purpose of the trip--they thought they would have more bargaining power by shopping for two cars together, with both families wanting a new car.
My 17-year-old cousin, D, was not too happy to be told that she was responsible for me for the day. (Not that I felt that I needed the supervision.)
D went off into the study, and I was watching TV in the living room. After I got bored, I went to see what she was doing. She was sitting on the couch reading a magazine, and didn't put the magazine down or acknoledge my presence when I came in. Her bare feet were propped up on the coffee table.
I knelt by her feet, and gave her sole a tickle, dragging one finger from the heel to the toes along the whole length of the sole. She didn't laugh, but her toes twitched. I did it again, and she yelled "Cut that out!" but didn't withdraw her feet.
When I did it a third time, she said "Now you're going to get it," and ran after me.
She soon caught me, and pinned me down, sitting on my chest with her knees on my upper arms. She furiously tickled my ribs and armpits. I shrieked and laughed. I loved it. 😀
After about two minutes, she said, "Now apologize for what you did."
I knew how to get what I wanted. I said, "No, I won't, and you can't make me."
She dug in again, and tickled and tickled for about ten minutes. I loved it, but she eventually stopped. She told me not to bother her again, and went into her own room to read.
I regret that I wasn't a good enough SAM (smart-a**ed masochist) to get her to tickle me all day long. 😀