My wife and I plan on buying iPhones soon. My sister has one that she bought a month ago and she gets a 25% discount on her entire at&t bill. We want to add our phones to my sisters account under a family plan. Will we need to show proof that we are family members? We have the same last name but different addresses. Will this be a problem? And when we go to at&t to buy our phones, should we take my sister with us so she can set up a family plan right there in the store and we can join it right then? Thanks!