>>534669681
The original heroine, who had Tifa's name and face, was siblings with Sephiroth. Original Sephiroth had Vincent's face. That's why Tifa and Vincent look like siblings. Aerith's design was created (reused from FFV's Lemure) when they decided to split the original heroine and kill the ancient. Aerith was designed to die, so they could preserve Tifa design and make her still ends up with Cloud.
Fun fact: Aerith's scenes were incorporated late in development. The developers added more screentime for her because some team members felt Tifa had too much.