While there's no evidence of this, I think you're on to something here. The proto-Aerith/Tifa hybrid character had Tifa's design, after all. In this stage Aerith and Sephiroth were related, so when they drafted a new look for her they designed them to be similar. That's why Aerith and Sephiroth have the same bangs. At this time Sephiroth also looked more like vincent, who arguably resembles Tifa streeeeetch so maybe that's part of the reason that design got repurposed in favor of Sephiroth's final look. This way the siblings look like each other and the Tifa design gets to have another role. I dunno, that kinda makes sense.
And I've seen a quote where Nomura jokes that he saved Tifa by giving them Aerith.