Story ID requested:
There's a post-nuclear Earth c. 2060 in which the 1962 Cuban Missile Crisis escalated all the way. A brother and sister are sent back to keep that from happening. They succeed, but nuclear war keeps breaking out at later times, repeatedly requiring their intervention. The story (which I don't want to spoil) has a bittersweet resolution.
Can't find any mention of this book anywhere. It was written in the late 80s, I think. Help?