How old is she and how old are you?
If she is over 18 it'd probably just be best to come out and ask her if she's into Bondage, Sibling-to-Sibling, then you could ask her if she will let you tie her up so you know what it's like...
If she's younger (Like under 12), you could play a game of truth or dare and dare her to let you tie her up... Then ask if she enjoyed it...
Anything else (Youll need to be pretty confident to do this) and you could just say that you wanted to try something kinky with your girlfriend and ask your sister if she'll let you try it on her...
Hope I helped

Tie Me Up if you want, but once I escape, you'd better watch your back 3:D
Feel free to chat anytime, just PM me
