The origins of the Kankers are convoluted. While they are definitively sisters and all live together, they appear to have three different fathers but only one mother. This seems to suggest that they are actually half-sisters, sharing a mother but each having a different father, which would explain the lack of physical resemblance between them and live in a trailer.
In the Series
They were introduced in "Nagged to Ed". They live in the "Park n' Flush" Trailer Park. Although they are obsessed with the Eds, they still torment, tease, and torture them, although usually in an exaggerated "lovey dovey" way. However, it didn't help that the Eds were particularly demanding of them in said first appearance though the Kankers' response still counts as Disproportionate Retribution especially as the series went on.
Many episodes conclude with the Eds encountering unwanted kisses from them or by harassing them. While all three sisters apparently love all of the Eds, May prefers Ed, Marie prefers Edd, and Lee prefers Eddy, though Eddy seems to be the most tormented of the three. For obvious reasons, the majority of the cul-de-sac seem to fear them. The Kankers' idea of a vacation is a trip with their trailer to Eddy's house. On other occasions, they have shown incredible strength and brutality, such as when they literally caused the Cul-De-Sac to break apart when searching for their lost ship-in-a-bottle (taken by a sleepwalking Ed), rendering it into a scene out of a post-apocalyptic film. However Rolf managed to be unaffected by this as he hid when he saw his farm animals sensing the coming danger.
The Kankers are essentially the equals of the Eds, who cycle through feelings of love and hate for them. Sometimes, they become manic possessive towards the Eds; other times, they will simply yearn for romance; and sometimes they simply abuse them. At first, the Kankers appeared to be kind-hearted, performing domestic services for the Eds such as cooking and washing clothes. However, when the Eds began to push their luck, the Kankers felt rejected and felt the compulsion to torment the Eds ever since.
The lives of the Kanker sisters apparently revolves around activities such as painting their toenails, stylizing their hair, and reading such literature as Hub Cap Digest. Their trailer holds a large amount of nautical-themed items, such as "Fish Bowl 2", a stuffed swordfish, an anchor, a diver's helmet, and a ship in a bottle, which is a prized possession. These sharply constrast with advertisements for car parts, a perfume of their own design called "Krankshaft No.5," and a penchant for wrestling each other.
Ed, Edd, n' Eddy's Big Picture Show
The Kanker Sisters act like anti-heroes in the film. During the film, the Kankers first appear when Jimmy and Sarah try to seek out the Eds with the hopes of watching them get beaten. The Kankers torment Jimmy and Sarah until they reveal the other Cul-De-Sac residents' intent to hunt down the Eds. Determined to save their "boyfriends", the Kankers force Jimmy and Sarah to cart them along the path earlier taken by the Eds. Sarah and Jimmy barely manage to escape the Kanker Sisters, leaving the three to relentless pursue them.
Along the way, the Kankers capture Kevin, Nazz, and Rolf in an attempt to "save" the Eds. Once they find the Eds at the trailer of Eddy's Brother, they prepare to assault the Eds once more until they're cut off by the appearance of Eddy's Brother. Like the other kids, the Kanker Sisters are horrified by Eddy's Brother's abusive treatment of Eddy, with May and Marie holding back an enraged Lee. After Eddy's Brother's defeat by Ed unhinging the screen door of the trailer, the Kankers are last seen dragging Eddy's Brother's unconscious body into the trailer with amorous intents.
A theory had emerged from the Internet that the Cul-De-Sac is Purgatory. Stating that all the children are dead, and that the Kanker Sisters are demons sent to torture the children.