The Savages (Tamara Jenkins, 2007) Maybe it's a pack-order kind of thing.
I've had enough conversations with children of aging parents complaining about their siblings who want nothing to do with the responsibility (and I usually end up talking to the sibling who enables them).
I say "pack-order" since, once out of the shadow of their parents' influence, the children suddenly see themselves as "top dog" in value, rather than having to be supplicant in the demands of their parents. Then, as time goes by, and the roles of need become reversed—the child must fend for the parent while that parent tries to assert the illusion of authority over the children (something reinforced in the kids by a lifetime of obedience, mixed with bitter defiance).
To disrupt this natural balance leads to the inevitable melt-down of the nuclear family. And both sides resist the change and hold on to their acquired ground, when it's as natural a part of the life-cycle as Death (which we also resist). It's as inevitable as a change in seasons, but it always seems to catch us by surprise.
The parent must learn (if they have a mind to) to accept their diminished capacity. The child must learn responsibility over another human being beyond their own needs. And both resist it, kicking and screaming, sometimes literally.
This is rich dramatic ground to till, and The Savages manages to break it down to its basic components. The two siblings (Philip Seymour Hoffman and Laura Linney—who would appear to have nothing in common other than their enormous talents) must deal with their diminished father (Philip Bosco, nicely delineating the edges of gruffness and helplessness), whose longtime companion in an assisted living facility has just died of a sudden heart attack, shattering the illusion of his independence. It's up to the kids, both academics and both lousy at relationships, to suddenly cope with unfamiliar territory: 1) real life and 2) the responsibility of being a partner. Of course, they fight like cats and dogs. Their rivalries, ostensibly for their parent's affection but also over the acquisition of writer's grants, is always churning just below the surface and bubbles up in passive-aggressive little blorps and clumps and repressed urges to kill (Linney and Hoffman excel at unspoken emotion—they wear it in their jaw-lines) and bitter sub-text.
Only a universal event can shock perspective into these two people looking inward for answers, and it is only the confrontation of common mortality that can civilize these savages.
The movie is funny and depressing as Hell.
But, that's Life.