I bet the form says that a parent has to sign the paper. What it does not say I believe is it has to be the dad or the mom specifically. But even then if you want to fake stuff like this use your parent's single name. If big bro had used the family name for his form and the single name for lil sis then they could have gotten away with it, but given that as the say "love makes people blind, stupid etc, etc" then here we have a huge nuke thrown at our beloved sibling couple.