Aeris was dead. Mind explaining how a dead person can cast magic on a living person? There is no real evidence to suggest she had the power to do...anything. The Lifestream was no where to be seen as far as I remember, they weren't at the church or any special place that could suggest her having contact with Cloud.
And, if we're saying FF characters can perform magic from beyond the grave, how would anyone win against the characters then? They could just hit you from beyond the grave with magic and nothing could stop that.