Aggression between siblings?

Question
I'm at a loss to explain the current behaviour of my sibling Siamese ,who are two and a half.

This behavior has been ocurring for about 6 months. Piper, a small lilac pt. is a total lush, very affectionate with both my husband and I. Her much larger choc. pt brother, Bandit, is more aloof, preferring to bestow his affections in a more subtle way. He also started spraying a few months ago and is now on Clomicalm which has been very effective. Holding court over both is my 14 yo lilac bruiser, Boris, who is deaf as a post, but who gets on exceedingly well with both of them. They both sleep happily with Boris, but not with each other, despite both devoted siblings when younger.

The spraying behaviour began around the time Bandit's previously doting sister began rebuffing his attempts at play with her hissing and swatting at him. And yet, when Piper decides to initiate play, Bandit joins in wholeheartedly, they chase each other madly then for no apparent reason, she hisses at him again and Bandit backs off, apparently afraid of her.
Is she trying to dominate him? Is Bandit being a bit rough with her? I just can't figure it out. I guessed that she was trying to usurp her older brother's apparent state of second in command, but that appears contrary to what I've read.
Any comments would be appreciated/

Answer
Christine,

I have two rules concerning the ways cats interact with each other.  (1)  Humans should never interfere. and (2) Humans do not understand the rules.

First off I am assuming everone is spayed and neutered.

Cat chase games are really a sign that they are getting along.  The fact that there is chasing going on even though it may end up in mock fights is a good sign.  If cats did not really get along, they would tend to avoid each other all the time.

It may be that Piper has decided to be an alpha female towards her brother.  It could have started out for a variety of reasons.  It could be a new addition to the family or a new cat in the neighborhood hanging around the house.  You would have to go back 6 months when the beahvior began.  At this point, there is probably little to be done since in the 6 months, the behavior has become ingrained.

I would just accept the new order and realize that cats figure out their pecking orders and, generally, are comfortable in their resepective nitches, even if it seems like a "vicitm" nitch.  Cats seem to have at least 3 pecking orders:  territory, feeding, and litter pans.  They could all be the same or different. I would not try to figure out what is going on or why, at this point.  They do seem comfortable in their newer roles.

Best regards... Norm.