I have no idea what Arrow thinks it is doing. At this point, I'm watching for Felicity. And I still fall asleep for the Gilligan's Island bits.

I agree the sister thing went by too quickly and was a total waste of River Song. I'd ordinarily think that this means there's more to it, but I actually don't respect the show enough to believe they're that smart. Not that the sister couldn't come back some day, but if she does, I feel like it will be because they're scrambling about for something to do, at some point in the future, and it won't be connected to this particular arc. That said, if I'm proven wrong I will be pleasantly surprised.