Dealing with a moody 5-year-old

Happy Candletime! This is the third year already. If you've forgotten the rules, they're here.

And now a question from K, who wanted me to summarize and paraphrase her larger request.

Essentially, her son, since turning 5, has gone all Jekyll and Hyde. "Sweet as can be, helpful, and polite vs surly (actual surly), rude, and vicious. Permanent getting up on the wrong side of the bed, if you will." He's vicious to his older sister when she tries to interact with him, and blames his bad moods and behavior on his sister.

K thinks it may be jealousy that she's able to read and write and do other things he can't yet. But will it go away? Is there something she could be doing?

I'm looking for data points from people who may have had the same issue. My own two boys were horrible at 4.75 years but changed radically into cooperative, fluent little citizens shortly after turning 5. So I had the opposite experience.

Is there anyone else out there going through a Horrible Fives phase, or who had a child who had one? How long did it last? Was there anything you did that you think helped?