- “How dare you disagree with us!”
- ―Aunt Sponge to James
After the death of James' parents, the two had no choice but to take in James and "raise" him. Instead of caring for him, they force him to do all the work around the house, they had a tendency to beat him when he disagrees with them or backtalk, and almost always send him to his room without dinner. They never call James by his real name; instead, they call him mean and insulting names like "rotten little grub" and "nasty little beast".
Aunt Sponge is a greedy, selfish, and morbidly fat woman, who is equally as cruel and repulsive as her sister Spiker, and presumably the younger of the two. Both she and Spiker are vain, each singing praises of their imagined beauty while they are in fact repulsive, but each usually ends up attacking the other's repulsiveness. Dominated by Spiker, Sponge is apparently a bit more dim-witted and gluttonous, thinking of eating the peach while Spiker seizes the moneymaking opportunities it will bring to make them rich. Sponge attempts to save her own life instead of Spiker's when she notices the giant peach rolling towards her. However, they trip up over each other and both meet the same end.
In the movie, after James' parents are eaten by a rhinoceros, he is sent to live with his mean aunts, Spiker and Sponge, who adopt him. In the beginning of the film, when James awakens from being asleep in his room, Aunt Spiker and Aunt Sponge come into his room, and Sponge calls him a "lazy little bug", and along with Spiker, orders him to get up and get to work. They are both very abusive and treat him like a slave, forcing him to do hard labor, beating him for no good reason, and feeding him nothing but fish heads. They call him offensive names instead of calling him by his actual name. They also have a terrible hatred of insects and bugs and kill them in a variety of awful ways, and are thus hated by them as well.
Later on in the movie, Aunt Sponge is seen sitting next to Aunt Spiker in a chair and tells her sister that if she takes off both her socks, she'll see her dainty toes, to which Spiker tells Sponge not to forget how much her tummy shows. Later, Sponge notices James reading his travel map to New York City and asks why he isn't working, to which James replies, "I've finished the wood." When she and her sister realize that James plans on leaving, Spiker and Sponge threaten their nephew by reminding him that the rhino will get him if he tries to leave, and Sponge tears up James' travel map. Both aunts then demand James to get back to work.
Sometime later, Spiker and Sponge discover a peach on a withered old tree (caused by the influence of the "crocodiles tongues" that James accidentally spilled when he received them earlier from a mysterious soldier) and watch it grow to immense proportions in a matter of seconds. Spiker and Sponge use the giant peach as an attraction, making lots of money as James watches from the house, not allowed to leave or play with the other children.
That night, Spiker and Sponge send James outside to pick up the garbage dropped by their customers. While doing so, he grabs a chunk of the peach as a "crocodile tongue" enters it for him to eat. James ventures into the giant fruit and transforms into a stop-motion animated character, where he finds and befriends a group of anthropomorphic invertebrate insects (who are all much larger than his small petite frame), who also dream of an ideal home. As they hear the aunts search for James, Centipede manages to cut the twig holding the giant peach to the tree, and the peach rolls away to the Atlantic Ocean with James and his insect friends inside it. One night, while navigating on the peach, James has a nightmare of him as a caterpillar being chased and attacked by Spiker, Sponge, and the rhino, eventually getting trapped in a blind alley before he awakens.
Later, when James arrives in New York City with the giant peach (having transformed back into his normal form by then), he is followed by his two persistent aunts, apparently somehow managing to drive their now-crushed car under the ocean and reach New York covered in seaweed and crabs. They then try to convince the authorities that James is a liar and that he be released to them. However, James, now no longer afraid of his cruel aunts, stands up to them and reveals the truth about their horrible treatment of him. Unable to believe that James dares to stand up to them, they furiously attack him with firemen's axes until the sudden reappearance of the insects come to James' rescue. James and the bugs then tie Spiker and Sponge up with Miss Spider's thread, and the beat cop has them taken away on a crane, after which they are presumably arrested.
After the credits, there's a scene showing an arcade-like game which consists of controlling a rhinoceros to attack replica models of the aunts.
In the book, when the peach gets loose, it rolls over Aunt Spiker and Aunt Sponge, instantly killing them both, and they subsequently do not reappear at the end.
In the 1996 film, they survive and pursue James to New York City in their beat-up car (in the Ice World, James and Miss Spider see a shipwreck with figureheads, having a terrible resemblance of the two aunts). Upon arriving in New York, both are soaked as if they had been driving their car across the seabed. They attempt to kill James but are tied up with Miss Spider's silk and arrested.
- When James and Miss Spider were searching for both Mr. Centipede and a compass, one of the sunken ships had both Aunt Spiker and Aunt Sponge on the mast for unknown reasons.
- Aunt Sponge has similarities to Lady Tremaine, as both of them force the protagonists of their respective films to work like slaves.
- Coincidentally, Spiker and Sponge also resemble Belle's sisters, in particular, the fat and lean one, respectively.
- Sponge's name is most likely derived from the slang word "sponge", meaning to absorb a lot of wealth without actually earning it.