The folktale which roughly translates as The Sparrow with the Split Tongue is a fairly traditional tale with a message or moral.
The woman is portrayed as cruel and uncaring whilst her husband is hardworking and kind.
...the sparrow becomes his beloved pet, much to the frustration of his greedy and trouble-making wife. She becomes jealous ...
He is also humble; hence, he chooses a small rather a large 'treasure' chest when it is offered to him. The small chest contains gold and jewels and, after his wife has finished shouting at him because he was away from home so long, she berates him for not choosing the large chest which presumably would have contained even more gold and jewels.
As an overbearing person she intends to claim her own 'treasure' chest and she visits the so-called 'sparrow maid,' who is apparently a transformation of the sparrow to which she was so cruel, demanding the same privileges shown to her husband.
She gets her just deserves and, having chosen a large chest, as you would expect - she finds it full of poisonous snakes. Ironically, they bite and kill her.
This is ironic because the very thing she expected to benefit from turns out to be her undoing. The irony extends to the fact that people are self-serving and often do not see their own faults.
The reader probably does expect that there will be something nasty in the chest as the wife has been portrayed as NOT deserving of the finer things in life due to her attitude and selfishness. She must get her just derserves.
The folktale "Tongue-Cut Sparrow," often translated as "The Sparrow with the Split/Slit Tongue," is a classic Japanese morality tale about greed and kindness. The old couple is rewarded in kind for their own actions, the man receiving both an enjoyable evening with his beloved sparrow-turned-human and a chest of gold, and the women receiving only snakes.
...she begged him quite politely to tell her where he had spent his evening, and how he had come by these wonderful riches. So he told her the whole story, and she listened with amazement, till he came to the choice which had been given him between the two chests. At this her tongue broke loose again, as she abused him for his folly in taking the little one...
("The Sparrow with the Slit Tongue," mythfolklore.net)
The ending is ironic because the actions of the old couple are echoed back on them. The man, who showed the sparrow kindness and never asked for anything in return, is allowed to choose a large or small chest; he chooses the smaller chest, presumably because he is old and doesn't want to carry the larger chest, and still find that it contains great wealth. The woman, who is cruel to the sparrow, goes chasing wealth and finds nothing but cruelty in return; the snakes are not even guilty of deliberate cruelty, but are simply acting according to their instincts. The man expects nothing and receives everything; the woman expects everything and receives nothing.