He intends to make sure pretty much every bit of her ached by the time he was finished. Use her so thoroughly she won't be forgetting this night for quite some time.

You're still so tight. Even after the amount of times I've had you so far tonight, much less all the times my brother and I have taken you over the last several months, he purrs in her head, ignoring that at the beginning, it was her having him instead. I wonder if making sure you'll always remain so was one of the modifications he made to you.