Sudden Fighting

Question
My husband and I have two cats, brother and sister cats just over 2 years old
(we've had them since they were 12 weeks).  They've never been separated,
and are generally well behaved.  They've always had playful fighting, but
never anything serious or prolonged.  Since last night, however, the female
has been stalking the male, then chasing and attacking him.  They are
hissing, growling, and screeching like I've never heard them do before.  It
ends with the male hiding away for a few hours.  We separated them last
night, but first thing this morning, she's stalking him again.  

The only thing we could think of was take them out back in the garden (we
have cat harnesses for them, but they aren't outdoor cats), and they got along
fine, but the second we came back inside, they were at it again.   Any
suggestions? Thank you!!

Answer
Maria,

My bet is that one or both of them saw a strange cat through a window and got into a fight/flight mode.  When this happens, one of them will actually forget they know the other.  Usually in a few days this passes.

I would ignore it and see if it all returns to normal in time.  They are not really going to hurt each other.

In the garden, neither cat is dominant.  In the house , one of them is, and until they forget about what ever triggered the behavior to begin with, things will be a bit tense at home.

Best regards... Norm.