Can hags still cast when members of their coven die?

Question

My last D&D session included a fight against a coven of sea hags. Hags have some interesting and a bit ambiguous rules about how the coven works. One of the rules was this:

Shared Spellcasting (Coven Only)

While all three members of a hag coven are within 30 feet of one another, they can each cast the following spells from the wizard's spell list but must share the spell slots among themselves[...]

The question is do all three of the hags have to be alive for this to still work? The party focused attacks on a single hag to reduce the threat, but would killing one member of a coven have stopped their spellcasting? Would killing two have stopped the last member from being considered a coven and being able to cast spells?

Answer 1

A dead hag cannot be a member of a coven

Hag Covens (MM page 176)

A coven consists of three hags[...]

Once a hag is killed it is no longer considered a creature, and becomes an object. That is supported in this answer, and these (non-offical, but still fairly authoritative to some) tweets.

A dead hag stops being simply a hag (for rules at least), and becomes an object (likely called a dead hag, or hag corpse) which cannot be part of a coven.

This also makes sense narratively, the remaining hags would have to reform the coven by finding a third hag, so that

any arguments between two hags can be settled by the third

As suggested by @erik this could be a great reason to allow hags to make death saves.

MONSTERS AND DEATH (PHB page 198)

Most DMs have a monster die the instant it drops to 0 hit points, rather than having it fall unconscious and make death saving throws. Mighty villains and special nonplayer characters are common exceptions; the DM might have them fall unconscious and follow the same rules as player characters

Just make this clear to the players in the way you describe them being reduced to zero hp, because it is unusual so they might not fully understand these hags work differently to most creatures.

Answer 2

Since three hags are required for a coven, and a coven is required for shared spellcasting, my instinct would be to say no. While each remaining hag can still use its at will spells, all three are required for shared spellcasting.

This presents an interesting tactic to the party, who may work out that disabling one hag is a ticket to triumph. However, the interruption of shared spellcasting can make the encounter much easier, so some rule-bending might go a long way.