The outing was canceled and no one apart from her sister, who wasn't up-to-date, came.
Yes, in the original sentence, the sister came to the canceled outing.
No one else came.
No one except for her sister came.
Her sister wasn't up-to-date. Her sister didn't know that the outing was cancelled. That is the reason that just the sister came. Everyone apart from her sister was up-to-date.
Yes, the nonrestrictive clause "who wasn't up-to-date" relates to the phrase "her sister". Her sister wasn't up-to-date.
If we make the change that you suggest, we get the opposite meaning and a conflicting message:
The outing was canceled and everyone apart from her sister, who wasn't up-to-date, came.
Now everyone who had enough reason to ignore the outing came, but the only person who didn't know that the outing was cancelled is also the only person who didn't try to attend. Everyone came, except for the one person who still had a reason to come. Everyone knew better, but they came anyway.
I think you'd rather say "no one except for her sister" when you mean "her sister and nobody else".