Wordpress plugin modification

the plugin: [login to view URL] My client runs a language center for children, and parents sign up their kids. Here\'s the specific example of what needs to happen with all of her discounts: 1. Priority registration discount - early registration discount (ability to adjust in back end) 2. Sibling discount - here is where it gets tricky (sort of) - A parent signs up a child for a class and wants to sign up a sibling as well. All I need is a checkbox that says \"check if this student is a sibling of a current student\" and a form field that allows the parent to enter the sibling name -- this action will discount another 5% from a single course signed up for OR the most expensive course in the cart (cart function included in plugin) if the parents signed up their kids for more than 1 course. To get the sibling discount, they have to check the sibling box. How: The plugin comes with a registration form creator that allows for checkboxes. You would just have to hook into that and set that if the sibling checkbox is entered into the form, then the specified discount applies. example: Let say, a sibling takes 3 classes, out of these 3 classes, the most expensive is $100, the second class full tuition is $90, the third class full tuition is $80. We will charge this sibling as follows: 1st class: $100 x 0.95 for his 5% off discount, 2nd class: $90 x 0.95 (5% sibling discount) x 0.95 (5% off extra class discount - already built in to plugin), 3rd class: $80 x 0.95 (5% sibling discount) x 0.95 (5% off extra class discount - already built in to plugin). The only thing that the plugin does out of the box is the sibling discount added through selecting the checkbox \'this child is a sibling of a current student\' or whatever the wording will be in the registration form.....it\'s a strange system, but I hope that made sense....let me know if you have any questions thank you!