I think the point of this sentence is to show that Ketkä is the plural form for "who." In English "who" can be either singular or plural, though its use in plural is dropping out. We could say "Who are involved in this affair?" but more likely "Who of you [who among you/us/them] are involved in this affair?"
The English sentence feels clumsy to my ear. I would more likely ask "Who's (who is) washing the dog" even if i knew there were more than one person. Having said that, I believe its technically correct, and as @Oinophilos mentions, it's demonstrating the plurality of the Finnish word
... true. But very odd english, though technically correct. Interestingly it would be fine if one were washing several dogs. It does't seem to follow size of washed object, because i'd also say"who is washing the mountain?" Unless you want to specify that lots of people are doing the job, who would be either contextually singular or plural, and still take is. Very interessting!