Honestly, Jesse gives almost no reaction at all. Here is what the text says about Jesse's reaction.
"Oh?" said Jesse, his eyes on Miles. "Where's the fish, then? How come we got nothing but flapjacks?"
Despite seeming to appear as no reaction at all, I think Jesse's reaction is feigned indifference. During the previous night, Jesse Tuck sneaked downstairs to talk with Winnie. He says a bunch of random things to Winnie, but he does convey two important things to her. First, he gives her the bottle of immortality spring water. Second, Jesse practically proposes to Winnie.
"But the thing is, you knowing about the water already, and living right next to it so's you could go there any time, well, listen, how'd it be if you was to wait till you're seventeen, same age as me—heck, that's only six years off—and then you could go and drink some, and then you could go away with me! We could get married, even."
I know this book is a fast moving, young adult novel, but that scene grosses me out every time. He's 17, she's 11. That's a high school junior talking about marriage to a fourth grader. Anyway, it's obvious that Jesse is smitten with Winnie. So when he comes to learn that his brother got to spend the entire morning alone with Winnie, it has to bother him. That's probably why Jesse focused all of his attention on Miles and responded with a thinly veiled insult about Miles not catching any fish.