It is play! Nothing too much to worry about, just keep an ear peeled. As lvpets2002 said, you will know if things get out of hand. Right now, kitten and cat are just establishing their places, and she has plenty of energy to burn, so play's the thing! If he seems particularly bothered, you could try burning off some of her energy with "fetch" or a "fishing pole" or other toys that will amuse her, but what could be more fun than "attack the brother?"