["I promised my sister that I would go sky diving with her when she was old enough to, but I didn't think she's remember. I'm horribly afraid of heights.", 'How long ago was it?', "I promised her about 8 years ago.  I was much younger also but now I have two kids and don't really want to do it.  Thinking about dying or even just jumping off a ladder, much less a plane, seems awful!"]
Maybe you can talk to her about it?
I promised my sister that I would go sky diving with her when she was old enough to, but I didn't think she's remember.  I'm horribly afraid of heights.