The Grape family's relationship, in general. They're definetly a Big, Screwed-Up Family, but despite their tiffs and differences, they look out for one another and clearly love each other deep down.
Gilbert reconciling with his mother during Arnie's birthday party. Exemplified when he insists that Becky meet her (after previously rejecting Becky's inquisitions on the subject), showing that he no longer feels any shame or resentment in caring for his family.
Arnie and Gilbert reuniting at the birthday party, with Amy playing along in their "Have you seen Arnie?" game.