The primary theme that the playwright explores is family conflict. He extends this theme to the dualistic love-hate relationships between generations and among siblings of the same generation. The “reunion” of the title, therefore, is ironic: although most of the family members are physically united, their actions before and during the party push them ever further apart. In addition, as Amy seems to be the family’s center, her death at the play’s end signals the dissolution rather than the reunion of the family and, by extension, of the Wishwood estate. No one in the family arrives at this party in a picture-perfect manner. Amy’s two youngest sons do not even show up to the estate—they have been slightly injured in motor accidents. Harry is haunted by his wife’s recent death. Mary is embarrassed that she is nearing thirty and remains unmarried. Even Amy’s sister Agatha harbors guilt for an affair she had with Amy’s husband many years ago. While seemingly picturesque, the facade of the reunion crumbles quickly.

The power of place is also a thematic undercurrent, as the physical estate assumes outsize importance in Amy’s scheme, which the younger characters variously endorse or reject. Amy attempts to “reunite” her family as the title of the work implies. She does not appear to want much more than physical proximity, however, as the birthday party quickly deteriorates with familial conflict. It seems that even when everyone is in one place, the family becomes less and less enchanted by Wishwood. Harry is a prominent example of this decline. As the eldest, it is Harry who is the acting Lord Monchensey. He will, presumably, rule over the estate upon his mother’s imminent death. However, Harry is not interested. He has experienced loss and no longer wishes to reside in Wishwood, especially after several years of being removed from it. The place itself lacks the ability to bring him in as Amy had hoped. Mary, too, wishes to remove herself from Wishwood. It seems that just as Amy wastes away in her old age, the appeal of the estate does, too.

The negative inspirations for and repercussions of many characters’ behavior suggest a theme that might be described as “what goes around, comes around.” In this regard as well, Eliot draws explicitly on the Greek tragedy by including the Eumenides, or Furies, who pursue those guilty of heinous—especially capital—crimes. Initially, it is Harry who sees them, foreshadowing his later admission that he believes he killed his wife. Because Agatha, Amy's sister, and Mary, a younger distant cousin, also see the creatures, Eliot suggests that they, too, may suffer under a burden of guilt. Although Agatha and Amy initially seem close, it is revealed that Agatha had an affair with her sister’s husband, Harry’s father; the husband had turned so far against his wife that he had plotted to murder her. Although Agatha had saved both her sister and the as-yet-unborn Harry, Amy cannot forgive her. Yet Downing, the family servant, also sees the Furies but is apparently innocent of all crime. It seems that guilt pervades the family reunion: there are many instances in which the characters reflect on their pasts and the repercussions they experience in the current day. The Family Reunion unites a family in proximity and draws together the consequences of the characters’ actions.