[pseudoclever]

A few things:
I still continue to be torn on the whole "little sister" thing. She started out eight years old, but apparently this is no longer the case. She also doesn't seem to be his sister anymore (?) but HE still knows she was. There are probably people who are going to freak the f out about this. I'm not one of them, though I would probably prefer if the incest angle wasn't part of the story. Like SM or mega though, these are aspects of a story I'm willing to put up with.

It's a nice twist that suddenly Rob can control the growth, and that perhaps he was doing it subconsciously earlier. Explains what happened to the machine.

Finally, it would be nice to get some "how tall is she," and "how big are her breasts" moments in there. You keep saying she's getting bigger, but there isn't really any perspective added except that she's pushing him off the sun chair. As in, she must've been seven feet tall, his head only came up to her navel, or whatever. It's hard to picture without this info.

Anyway, good job.