I have a two ideas that both involve patches.
One: make Chiaki 18 years old to make it ok on steam, and make a separate patch that makes her 12 with lewd content.
Two: keep her 12 and don't add lewd content, but have a separate patch that puts the lewd content in. The second one is probably easier since you can just fill in purposely made blanks with a patch rather than changing the whole animation and making two animations for 1 character. You won't see the lewd scenes in the original but you'll get the extra scenes with the patc