"What one parrot has, the other one has to have it too." That's a maxim I'm sure anyone with more than one parrot is well aware of, and it applies whether or not it's something the parrot likes. Ralph's favorite food is fresh green peas, which Scooter the CAG doesn't care for. When Ralph is sitting on his cage while I'm preparing the birds' breakfast, I often give him half a pod with a few peas in it. Of course, Scooter noticed that and now wants (and expects) half a peapod of her own. She graciously accepts the peapod, pulls out the peas and drops them to the floor and rips up the pod. She has a wonderful time doing that. But eat a pea? Never!
They are fascinating creatures. As I've commented before, Bonnie is completely disinterested in many foods I put in her food dish. But if I "happen" to leave it on the kitchen bench while she is out and I'm preparing food she will try all sorts of things (as she thinks she's stealing it, so far more interesting!). As I don't have more than one parrot inside normally - although we have a Bourke's parrot inside in a cage due to last Saturday's cyclone - I don't know about the sibling rivalry thing. Bonnie does love to rip up Iceberg lettuce just as an exercise. I don't think she eats any of it.