Thanks to natural selection, women need not actually (as widely believed) have to have delivered a baby to lactate (i.e., make breastmilk). A back-up mechanism evolved which allows a woman to make breastmilk after prolonged (as long as a few months in some cases) nipple stimulation--likely made so by adoptive mothers in prehistoric times who had their nipples sucked on by orphaned babies, produced breastmilk and thus helped the orphans survive (this is called induced lactation).
A side-effect of this is (usually) completely natural breast enlargement.
Or it could just be that Akiha's hormones took that last step. Puberty doesn't work step by step.
Aggh I don't care if they're brother and sister they're not related by blood anyways! It kinda upsets me when the restriction is just the ONE tiny little thing. I really liked her H-scene in Tsukihime second to Kohaku's. It was really sweet
But this would be a totally perfect scene though. Aww it's so cute. Love it
And I've seen the CGs to Tsukihime. In my opinion, one of the best is when Shiki shows Kohaku some tenderness, unties her kimono, and actually (unlike many h-game protagonists I've seen) tries to please her. It's so much more erotic when the participants actually please one another rather than one just using another.
Not that Akiha's isn't tender either. I just find it odd that after telling the truth of Shiki's heritage to him, she stills screams out "Nii-san!" when Shiki first penetrates her.