At the part, where the Kankers say that they changed the Eds's clothes, each Ed looks at their tag. The Kankers say that they are their dad's. But there are three different male names!
Perhaps this is a reasonable slip-up, and I have realized that the "Canker Sisters" term is used often, yet perhaps they are not sisters. Not that they are nuns or anything, but I have seen teenage women use the term sisters to each other, even when they are not the same. In conclusion, perhaps they are just so poor they all live together, each sister and father, and that is why there are three different bath robes.
No..Keri has it right! The whole "joke" with the Kankers is that they have the same mom but different fathers. That explains why one is a redhead, one is a blonde and one is a brunette.