Iam an only child, but as I saw other family members mentioned in this thread, I thought I'd add to it.
I have two aunts, and when I was younger, say pre teen years, when one aunt was mid 30's and the other one was early 40's, I tickled their bare feet quite a few times. The experiences were over a short period of time, say a year, and it happened I would say, several times. The older aunt had extremely ticklish feet, and couldnt stand to have her feet touched or tickled, while the one who was in her mid 30's at the time, was what I would call moderately ticklish, but didnt freak out if someone tickled her feet.
I know this thread is mainly about brothers and sisters, but since I saw other family members mentioned in these posts, I thought I would add mine to it as well.
MitchellR