brother and sister cats fighting

Question
Hello.  Last night my two cats (brother and sister 3 years old) were enjoying the fresh air coming in from an open window at my house.  One minute they were fine and the next they were both hitting the screen in the window, knocking it out of place.  They then both went running after each other hitting and hissing at each other.  The female cat is really shaken up and barricaded herself in my laundry room howling, shaking, and even defecating when she saw her brother.  I separated them last night, and she seemed better this morning until she saw him.  What can I do?  They always got along and this has never happened.  Any suggestions are welcomed!  Thanks!

Answer
Is it possible that there was a stray animal outside the window when this happened? Maybe it scared them and/or tried to attack them?  You can ask your vet about the Feliway product
and see if that helps
Also try to come up with the source that caused them both to freak out  That might clue you in as to what is going on too