Man... fuck Steam.
Would it be possible to keep the current design and just make up some bullshit to explain that she's actually 18? Maybe she's got a condition that stunted her growth. Maybe that could even be the reason her brother is so protective of her. There are plenty of 18+ steam games with lewd content of characters that look even smaller than current Chiaki.
If you really think it's necessary to age her up though, go for it. It's disappointing, but I think this will still be a great game even with the change. Though I can't help but feel that h-gaming as a whole is losing something from having game devs bow to Steam like this.
In the event that aging up Chiaki still doesn't lift the ban, can we go back to current Chiaki?