My favourite TUG position is being staked out spread eagle on my bed or the back garden (see "When the cat's away......). Although the ropes are usually tied really tight, and the limbs might go stiff/number after a while, its doesn't hurt like the chains in the cellar because no part of the body is defying gravity.
In "When the cat's away.....", Gemma stayed with me and we had some real quality brother-sister time just chatting.
If it is KT (Katie - my girlfriend) who is the captor, she will often sit or lay on top of me and we just chat or kiss (sorry if you're being sick! LOL), but we are NOT having sex as we are not married yet.
One stake-out I particular enjoy is where Gemma stakes me out on the grass, but instead of staying with me as she did in "When the cat's away.....", she sits in the conservatory to do school/uni work. I'm not completely alone as i'm still in her line of sight, but I am on my own on the lawn. She usually "hoods" me with a school gym bag, and although they do get uncomfortable/unpleasant after a while (being made of nylon), they enhance that feeling of "outdoor isolation", even though I know I'm not totally isolated and Gemma is only a few metres away. Being staked out on the grass is very restful and (especially when hooded) I can drift off into an imaginary TUG world or even just go to sleep!
Paulscot
So, I can fully understand your sister if she likes to do that to you 
