Well, I figure…if it’s true, maybe we helped a little. If it’s false, no harm done. I mean the OP can go away chortling, but we, as mighty Dopers, feel no sting or anger.
We’re too good for that. 
It’s amazingly fucked-up. Don’t let the ‘If it feels good, do it!’ crowd fool you, engaging in bondage and whatnot with your sister is not healthy behavior.
Still, I strongly suspect another motive to your outlandish post.
When I first started reading the replies, I thought that I’d have to be the first to disagree. You’re free to do what you want, but in my opinion, the whole situation is weird, and you should stop. I’ll just mention now that I don’t have a problem with what you’re doing, but who you’re doing it with.
This is what bothers me. Sounds like you and your sister have serious self-image problems, maybe need counseling and surely need to get out with friends and get some hobbies and interests that will help your self-esteem.
I’ve heard weirder things, though not many.
Like others, I worry that this may be hurting both of your chances for finding and keeping romantic relationships in the future. But if you and your sister would really love nothing better than to continue with things exactly how they are, then… well, okay, then it’s still pretty weird.
I wouldn’t be at all surprised.
I’m closing this thread because it’s in the wrong forum, the title is too vague, and it’s way too much information to be sharing in your first post.
Lynn
For the Straight Dope