Natalie Babbitt encourages us to think of all the Tucks as foils for each other. After all, they're all in the same situation—i.e. they're immortal—but they all handle that situation differently. Our author writes, "The Tuck family has four members, and they were chosen specifically to talk about different points of view of living forever" (source). Having four different viewpoints highlights each of them as unique and distinct, don't you think?
We're going to focus for a second on Jesse and Miles, brothers and foils extraordinaire. Let's take a look.
Jesse never got married, so he didn't have to give up much in the love-department when he became immortal. Miles, on the other hand, had to give up his wife and his children. Now Jesse seems to enjoy immortal life, even encouraging Winnie to join the club, while Miles is less excited about the whole shebang. These differences are reflected in their appearances, too:
[Miles's] face was like Jesse's, and yet not like. It was thinner, without Jesse's round cheeks, and paler […] He looked solid, like an oar, whereas Jesse—well, she decided, Jesse was like water: thin, and quick. (17.11)
Notice the differences? Oh, and P.S. "quick" can actually mean "alive" (source). What do you think—is Jesse more alive than Miles?