Tonight, to my delight, I learned that Éowyn has changed all this. After giving both dogs some cuddle time and play time after I returned from class, I grabbed the dog brushes from my dresser. Normally, Jasper slinks off to hide in another room or under the bed, waiting until I call him, then he slinks back as slowly as his desire to be obedient will allow. Not so tonight. He came right to me, slowly wagging his tail, then stood gazing at me with worship while I brushed his coat until that nice shine was restored. Then, when I went to brush Éowyn, Jasper actually came to me acting like he wanted more brushing. I gave him a bit more, plus a treat.
I'm pretty sure I know what's on that little boy's mind. He's accepted the new sister, but he misses having alone time with "Mummy" and recognizes the opportunity that brushing gives him. He just wants to still be the special boy, and I keep reassuring him that he is.
I also suspect that he was also taking the opportunity to once again prove that he's the "good one." I've noticed that Jasper has been even prompter to come when called, faster outdoors, and quicker at playing fetch . . . all since yesterday. Since Éowyn is still learning how to be a good dog, Jasper's making sure that everyone knows that HE is ALREADY trained.