Ben Ockrent’s black comedy about a family in mourning has distinct strains of the ludicrous, though the cartoonish absurdities creep in gradually. But then families are ludicrous: badly behaved and falling into their early, childish roles, especially in extremis.

The extreme situation here is not the recent death of a mother in itself, which has led four adult siblings to gather at her home and hash out matters of inheritance, but a single item passed down by their grandfather which brings them to blows.

It is a painting, probably by the impressionist master Camille Pissarro, and also probably stolen from a Jewish household whose members perished in a concentration camp during the second world war. The siblings don’t know how it got here but suspect it was swiped by their grandfather – a medalled war hero who served on the German front.

New knowledge of this multi-million pound family heirloom sparks moral monstrousness in them all: the rapacious Jonny (JJ Field) has already made plans to sell it on the black market while the eldest and bossiest sister, Liv (Sally Phillips), is adamant that they will not benefit from Nazi treasure (“We are good people”). Younger siblings Michelle (Charly Clive), a primary school teacher who is accused of acting like a child herself, and fragile flibbertygibbert Rob (Sam Swainsbury), take their various positions around the eldest two.

The setup is not unlike that in Branden Jacobs Jenkins’ Appropriate, in which a toxic inheritance is found in the proverbial attic after the death of a parent. That drama – about race and toxic slavery legacies – has more punch. Here the moral questions threaten to overwhelm Ockrent’s script.

Under the direction of Michael Longhurst, it is rather subdued and stilted at first, playing out on Joanna Scotcher’s plaintively half-packed living room with debating society arguments around the ethics of keeping the painting, or otherwise. The four characters remain unknown and Phillips gives an initially understated performance, the others following, it seems.

But the tone rises to a crescendo and so does the physical comedy, which is so ridiculous and yet so funny, involving everything from their mother’s amethyst healing stone (turned into a weapon) to an ancient tree, felled in anger and dragged into the house.

It is too artificially resolved – you do not believe in the switch to sibling tenderness at the end. But even so, the whole thing is riotous, ridiculous, and hugely entertaining, while carrying the resentments and the ugliness that grief can lay bare within families.

• At Lyric Hammersmith, London, until 18 July