My sibling cats are not getting along

Hello! I have two sibling cats, male and female, both neutered. They will be 5 years this years and have always had a good relationship. However, since we moved houses, there are sometimes when the female cat doesn't seem to recognise her brother, and is aggressive towards him. I associate this with the fact that there are outdoor cats where we live and the do come by the window and sometimes even pee there. Is there something i can do? Please, help me  

Hello!  To give you some background, cats are quite solitary animals.  Dogs and humans are very good at sharing territory and using facial expressions etc to settle disputes.  Cats, as a species, are not so good at this; they tend to live solitarily for much of the time.  They have their own territories and enforce the borders by toileting and going almost straight into fight-mode to settle any encroachment.  Tolerant situations can be reached of course, especially where siblings have grown together.  But suddenly, there has been a complete change of environment here.   It is possible that the cats now have to define their territories again and it stands to reason that conflicts will come up, especially at the borders.   My understanding is that each cat has its own separate territory within a house eg one lies in this place and another in that, and can happily skirt round each other when territories are clear.   Drawing a plan of your house and marking (in a different colour per cat) where they hanging out, several times a day, and where the fights start, may help you to see whether this pattern is true for yours.   The path to food and to the exits are often shared and / or fought over, and furthermore litter trays or urination sites are a prized resource.   It is generally recommended that each cat has a choice of toileting sites as the toileting of other cats close to their territory can be upsetting for them (we were told at college, a litter tray per cat, plus one extra).   I know that owners sometimes hate to do this when their cats are already housetrained, but each cat having a tray and feeding station on their own territory can sometimes help to keep things calm.   As with any new arrangement, a certain amount of time may be required before making a judgement each time.    However, this is only a sketchy overview; your vet or a cat behaviourist (chose one with suitable qualifications, as recommend by your vet, as behaviourists are not regulated) may be able to help you to understand what is going on.   Please do keep us updated.