coven and ethereal plane

[da newt]

So a party is fighting a coven of night hags, and one of the hags leaves the material plane for the ethereal plane - how far away are they?

In order for the hags to use their coven spell casting, they must all be within 30' of each other.

Where is the ethereal plane - does it occupy the same space as the material plane?

[Warder]

The Border Ethereal overlaps and touches the Material Plane, but it's still a different plane, and there is no meaningful way to measure distance between two planes. I'd personally say that no, they're not close enough to maintain contact, like I'd say that someone can't just Misty Step out of the Border Ethereal even though they can see the Material, but it wouldn't be too egregious if a DM ruled otherwise.

[Grod_The_Giant]

I would say yes, they probably are supposed to be ae to still use their coven spellcasting, just because that's the most aggravating answer and everything about night hags is designed to be maximally aggrivating.

(No, I didn't just have a run-in with night hags in a game, shut up)

[Thunderous Mojo]

The Border Ethereal overlaps and touches the Material Plane, but it's still a different plane, and there is no meaningful way to measure distance between two planes.

The DMG measures the distance in feet, as stated here:
From the Border Ethereal, a traveler can see into whatever plane it overlaps, but that plane appears muted and indistinct, its colors blurring into each other and
its edges turning fuzzy. Ethereal denizens watch the plane as though peering through distorted and frosted glass, and can't see anything beyond 30 feet into the other plane. Conversely, the Ethereal Plane is usually invisible to those on the overlapped planes, except with the aid of magic.

The DMG also states that force effects extend to the border ethereal, and two living creatures can not occupy the same space at the same time, even if one is on the prime material plane, and the other is in the border ethereal.

A halfling can share the square of a huge creature on the material plane, but not when Etherealness is added to the equation.

A character with Truesight and the Magic Missile spell could attack a Night Hag in the Border Ethereal, from the prime material. There is a reason that a Night Hag can cast Magic Missile at will...that reason is some dastardly tactics are enabled.