well thats a tricky one isn't it.
if she knows what you guys went thru i am sure she'd find out for herself if it was possible for her to have any kids ? maybe?
maybe you could casually bring it up, and say " have you ever thought of being tested to see if you can have kids" at least you'd know and if it is a touchy subjest then you know what not to say.

i really don't have an answer for you, but she's your sister and ususlly they are very forgiving. good luck.