The multiple sibling issue is one that I encountered this quarter. I have 2 students with the same parent. Neither student has their own email address, therefore I have them put their parents email address in to register. Unfortunately, the second child can't do this because his brother already used the email address.
The functionality that you describe would be AWESOME!!!! I could definately sell Moodle to my administration and teachers with functionality like this. Our number one complaint form parents is the lack of communication. We need something like this to simplify that link.
Thanks to all the developers that make Moodle what it is; the best Open Source LMS out there!!!