Male cat aggressive to sister cat

I have two cats, Gus and Rosie, who are siblings and were a bonded pair. For the last year or so especially, Gus has been aggressive to Rosie, not all the time but certain points in the night will get territorial and chase her off the bed or sometimes when I can tell he feels restless he'll go after her. Gus has always had a lot of energy--my mom used to say he had "crazy eyes", which I always refuted, but I have to admit that even at 4 years old, he still seems either anxious or has too much energy. Playing with him before bed doesn't seem to work... I have gone to the vet and explained this and they've asked that I do a full panel on Gus, which I'm happy to do but he's otherwise extremely healthy--he just is like a German Shepherd in a cat's body, lol. Any advice on this? Do you also suggest a full panel would reveal this or should I be looking more in the direction of a behavioral specialist? Thanks!

Hello!  That's an excellent question.  You seem to be asking if there is anything that would fit with this behavioural change that would actually show up on routine blood tests.  I can't think of an obvious example, but you could ask your vet directly, outright - they would usually know what sort of thing they are looking for.   Behaviourists do often receive referrals that aren't really about behaviour eg if an animal is in pain (there is no blood test for pain though), or  hyperthyroid (this might actually show up as you described, although 4 is young for hyperthyroidism).  Some liver diseases may be possibilities.   The vets' reasoning may be that if one doesn't look for these problems, the behaviourist could end up trying to treat an ill cat using behavioural methods when they need eg pain relief or a liver diet.  Some behaviourists may ask that routine bloods are done before presentation - it is not an ideal screening tool but might give some clues. I look forward to hearing what is concluded - will you keep us informed?