I have seen another way to revive Aeris, but the problem is that you need 16 mastered Life materias. Then, go to Ancient City, after going to that water thing, go to Rocket Town (make sure Cid and Tifa are in your team). Then, talk to someone (forgotten who you had to talk to). You should get an item and then, head back to the Water Statue. Ypu have revived Aeris and all of your 16 mastered materias disappear. Also, Aeris' level is the same as Cloud's (e.g. Cloud's level is 99, then Aeris' level is 99). I havn't tried this yet, but I will try to do this over the school holidays!