Can channel divinity affect a fiend in the ethereal plane?

Question

In a fight with a Night Hag coven one hag uses the Heartstone to become ethereal, I know that divine sense can't reach into the ethereal plane and sense her. But can a Channel Divinity affect her?

For example, the Oath of the Ancients paladin has this Channel Divinity option:

Turn the Faithless. You can use your Channel Divinity to utter ancient words that are painful for fey and fiends to hear. As an action, you present your holy symbol, and each fey or fiend within 30 feet of you that can hear you must make a Wisdom saving throw. On a failed save, the creature is turned for 1 minute or until it takes damage.

A turned creature must spend its turns trying to move as far away from you as it can, and it can’t willingly move to a space within 30 feet of you. It also can’t take reactions. For its action, it can use only the Dash action or try to escape from an effect that prevents it from moving. If there’s nowhere to move, the creature can use the Dodge action.

If the creature’s true form is concealed by an illusion, shapeshifting, or other effect, that form is revealed while it is turned.

Will it make the hag turned?

Answer 1

The Ehtereal Plane is not within 30 feet of you.

Turn the faithless has two conditions: “within 30 feet” and “can hear you”.

While it is likely that the hag can hear you, the hag is on a different plane, so the hag is not within 30 feet. The Dungeon Master's Guide explains:

The Ethereal Plane is a misty, fog-bound dimension. Its “shores,” called the Border Ethereal, overlap the Material Plane and the Inner Planes, so that every location on those planes has a corresponding location on the Ethereal Plane.

While each point in the material has a corresponding point in the Ethereal, they are not the same locations, they are entirely different with no meaningful definition of distance between them.

See this answer for an in depth discussion of the relationship between the ethereal and the material planes.

Answer 2

Turn the Faithless does not state that it affects creatures on the Ethereal Plane, so it doesn't

Generally, class features only do what they say they do, in the same way that spells only do what they say they do. This Channel Divinity option does not state how it interacts with creatures on other planes, so the default assumption is that it doesn't interact with creatures on other planes; the hag must be within 30 feet of the paladin on the same plane.

Although the Ethereal Plane is slightly different to how most other planes work, it is still it's own plane, separate from the Material Plane (on which I assume the paladin to be on for this question). If the Night Hag is "hiding" in the Ethereal Plane, then she is effectively able to avoid any effects (like Turn the Faithless) unless the feature otherwise states that it can affect creatures on the Ethereal Plane.

Borrowing from this answer of the other question you linked, rules designer Jeremy Crawford unofficially confirms how distances are calculated between different planes on Twitter:

If two people are on different planes of existence, they are infinitely far away from each other. For example, if I'm on the Material Plane and you're on the Ethereal Plane, we're not within 30 feet of each other.

In short, things on different planes aren't really any specified distance from one another, but rather an infinite distance - there's no way to calculate the distance between points on different planes.