In the U.S. we would say "in the sink", not "inside the sink". To me , "inside" suggests an enclosed space, such as a room or a box. "In" would be a better translation for how the sentence is really said.
That's absolutely right. However, they're also emphasizing that the Hebrew preposition is b'toch and not b'. If they wrote "in the sink" in English, we'd be tempted to translate it back into Hebrew using b'.
With all due respect to LSadun's comment, would there actually be anything wrong with saying ?נמצאים בכיור... Or even just הכלים המלוכלכים בכיור? Both sound fine to me, but I'd appreciate the opinion of a native or expert speaker.