In a sidewall, there isn't a lot of there there to hold everything together. If the nail went through the sidewall twice, that's an awful lot of damage in a relatively small area. If the nail sliced any of the sidewall threads (and you would see the end of the thread around the hole it did), replace the tire without question. Even if you decide to keep it, boot it and watch it closely, and if the holes get any bigger, replace the tire.