a lot of these diseases crop up. but yes,my sister got it when she was about 6, and I was 8, exposed and never got it. Of course this was in the early 1960's
What i noticed is that they seem NASTIER now than they did back when i was a kid. everyone i knew had chickenpox,but when my sons got it, it was a much more serious ailment, laying them both flat for a week.
i hope your sister is ok...