gswork wrote:

We have two lovely older (nearly 9) rescue dogs, male and female
siblings (dark haired lab crossbreeds). We were told that the female
only shows aggression occasionally, notably when her brother is under
threat from other dogs.

They sound like great dogs!

I see protective behaviors among my dogs. Gordo, my male chihuahua,
threatens Oppie, my male lab, whenever Oppie seems to be annoying Chile
(female chihuahua, littermate of Gordo). Oppie likes to get his nose
under her butt and lift it up for a good sniff. Gordo barks and lunges
at him, without making contact. Sometimes Gordo, snoozing under a
blanket, will hear Chile mildly protesting Oppie's rudeness, and he will
bark without bestirring himself to lift the covers.

FurPaw