They both occupy the same birth position (third child and also youngest), and have a special delight in each other.
If you ask Phoebe how old she is, she will likely tell you that she's three. She's two. And if you confiscate the apple from her that she sneaked from the fruit bowl while you were on the phone and then wrote a note to the apple-eaters in the household to eat the apple that Phoebe started, all while she is screaming on your hip, she might lean down as you write and say unexpectedly, "I see Phoebe" because she recognizes her name in print. That Phoebe. She's our darling.