My stepsister was my same age: and yes, she enjoyd a lot my humiliating punishments!
One time she even invited her boyfriend as a witness: I still remember the grin on their faces while I was taking my briefs off, then remainig completely naked...
My stepbrothers were instead one one year yonger and the other one year older than me.
Yes, you are right: it was a total loss of dignity for me. Tied, naked, on display for everybody to see, with the whip merciless doing its job: can you imagine that?