Why are my 2 westies fighting?

My 2 Westies - brother & sister, 6 yrs old & both spayed - have suddenly started to fight. Not constantly but the sister will suddenly act all cowed though the brother has done nothing or been near her. Then there will be a standoff, they will either have a lick or they will suddenly kick off. They've never been parted from each other and normally are completely happy together. They've only ever eaten out of the same bowl and slept in the same bed even when alternatives have been there. My partner and his son have recently moved into the house but I still give them equal amounts of lots of fuss....so I'm really baffled! And I desperately don't want to separate them or lose one of them at all.