I recently got myself into a bit of a pickle by setting in motion the process of an NPC (sister of a PC) transforming into a hag.

The setup is that two hags in a coven got together under the full moons to replace their dead hag sister with the NPC.  The ritial has already happened but the transformation itself I have set to take a while. 

So now my question is: how can the PC's stop it? Obviously they can always fail, but I want to give them sufficient ways to succeed. 

I was thinking something like a "cleansing" ritual or something. Any ideas/suggestions are welcome!

Your world, your rules. But simple answer is, you don't. But you need an answer for your PC, so  it's possible, but really hard and at times it will get gritty.

Per the lore and info on the monster entries, Green Hags eat a newborn and give birth to a new Hag "Sister". I assume that didn't happen.  Annis and Bheur Hags are Fey creatures so that gives us some room. Night Hags are Fiends, exiled from the Feywild. End of the day, you have to be "born" a Hag, per the literature. This means, we're goin off book here. Way off book.

So the real question is how did you transform the NPC? Was it with Reincarnate, which the broken coven might not have the power to access. Maybe through True Polymorph, which (witch... get it...) they might not have had access to at all (9th level spell). Did they use a homebrewed Cauldron of Rebirth, that casts that spell on a creature that is killed and then placed in the cauldron for 24Hrs and then reborn as a "Sister" of the Coven. Did the two Hag Sisters implant the soul of their dead Sister into the body of the NPC and now you have a "Lazarus Effect" scenario playing out? Something that is akin to a possession maybe?

However you decided that the transformation takes place, I would suggest that you use a spell or magic item that already exists, with a couple tweaks to fit the story, but the mechanics are already baked in for the reversal. For the Raise Dead or Reincarnate solution, the body (vessel of the Sister) would have to become lifeless and the NPCs soul would need to re-inhabit the body to become the PCs sister again. Yes, I'm suggesting that the PC be given the option of killing their sister to save her soul. Mindf@$k, I know.  Kinda like it. Might use that one myself.

I might also include maybe including some classic favorites as Hallow, Bless, Ceremony, Dispel Evil and Good, Magic Circle, Protection from Evil and Good... I think you get the idea. This might need the help of an Exorcist, that might require the party go on a quest to find a MacGuffin or 7 to bring the NPC back.  

What about some form of Remove Curse that has to be performed by a specific NPC? Like, say, the party has to take the sister to find a particular Platinum Dragon who is milling about as a humanoid (an almost Jesus-like character), or someone who is very high ranking in a particular deity's church?

The best way to make up magical cures as a DM is with a ritual. Let magically inclined PC's or helpful NPC's determine that the only way to reverse the transformation is to scatter the venom of a chimera around a ritual circle, light magic candles under the stars of a certain constellation, and burn the eyes of a hag in a brazier of hot coals while the npc stands in the center of the circle. 

The key to a good ritual is to make the components something the players need to go on an adventure to find. The natural time constraints of the sister's transformation provides some nice tension for the group, and if you want the players to be able to fail you can weigh combat encounters accordingly.

PCs kill both of the other hags.

PCs discover a counter-ritual. 

The sister has to be fully immersed in holy water. If you want to make it harder, or easier, make it be created by a cleric of a specific god.

The sister has to eat the roots of a rare plant from someplace far away. PCs either go get it, or buy it from the nearby herbalist who accepts payment as a future favor they owe her.

Off-book for the most part. I was inspired by the Three Hags of Tepest from Ravenloft, where afaik they became hags essentially by a mixture of trauma and magic and evil deeds (and possibly the meddling of fey, since they were said to be "gifts" from the faeries). The NPC was already on her way to become a hag, in this sense, and is becoming the new "sister" herself (no reincarnation of the previous hag or anything). Basically, the NPC has become so twisted in her mind that it is starting to affect her physically, and now with this ritual (which she did willingly), she has fully given in to the proverbial darkness.

I like all of the options you've given, especially the having to kill the sister bit (because a little mindfkery is fun!). Or remove curse that has to be done by the pc sister under the same conditions as the ritual. Quest time! Thank you!

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Um I say really long and costly cleansing ritual that is needlessly expensive and resource heavy.

Quote from Kaavel >>

End of the day, you have to be "born" a Hag, per the literature. 

 rules lawyering here, but if you happen to be a Hexblood, a hag can preform a ritual(if you are willing) to transform you into a hag