Sometimes family can be hard to relate to (pun intended). See, in his family, Jess is the odd one out – faced with his mom's figurative and dad's literal absences, and his older sisters' shallow obsession with appearances, Jess's ambition and desire to move beyond what he is marks him as different. May Belle's the only one much like Jess, but at seven years old, seems too little to be an ally or a friend. Leslie becomes his family instead. But when he loses her at the end of Bridge to Terabithia, Jess feels a responsibility to recreate and hold on to the kind of relationship he had with her. That might be one reason he reaches out to May Belle and finally invites her in to Terabithia with him.
In comparing Mrs. Aarons and Mrs. Burke, we see that there's no such thing as a perfect mother: even the most present parents have flaws, and even the most neglectful ones have strengths.
Jess's strained relationships with his four very different sisters show that blood isn't thicker than water: in other words, just because you're related to someone doesn't mean you understand, empathize with, or are compatible with them.