As Shifty said, Niva dies of radiation poisoning.
Future Industries tries to build a fusion reactor and because of cost cutting measures the project turns bad. Put into Lily's own words; Niva has to deal with the Avatar version of Chernobyl. When the reactor melts and people have been evacuated, she helps contain it an prevents the radiation from spreading aroud the world with airbending. Then she uses earth bending, lava bending and metal bending to ''utterly'' encase and entomb it.
Niva ends up being as irradiated as
the elephant foot. Because she ends up working so close to it, her body ends up being radioactive itself.
The. Elephant. Food.
First she kills the board of directors by giving them a fatal dose of radiation,

then goes into isolation and dies three years later due to radiation poisoning.
When she dies, her grave has signs saying not to get too close.
Chat: Yikes.
Lily: Actually, Niva's fine with that. By this point Lavi's been dead for 15 years. She meditated into the spirit world and after three years, when the effects of the radiation poisoning vecome too much, Niva also meditates into the spirit world and they are reunited. So, Niva is happy.
The fuck does this even mean?
Why is she so fascinated with how her characters'll die? Why you choose for your character to succumb to one of the worst ways to die, known to man?
Before dying, Niva writes books for her successor about philosophy and how the Avatar relates to other people.
Chat: Do the books she wrote become radioactive?
Lily: They don't. *pause* Does paper absorb radiation?
It's amazing how Lily can come up with with outlandish stprylike this, and then does absolutely nothing with it.
Niva has trouble learning earth bending. She learns sand bending first, because it is more similar to water and air bending, which is closer to her personality. Lily gives no info about how Niva overcome her difficulties earth bending.
Fire bending; she had no trouble with, because she is angry at and has resentments against many people. Which in Lily's world, anger is same as passion.
Air bending; she can't manage to learn, again in Lily's words, how to fly. In order to learn that, she needs to let go of her earthly attachements, which would be Lavi and Niva can't do that. She has an easier time learning to travel in the spiritual world because it involves the least amount of conflict. She does, however, clash with the air nomads and their culture a lot, because she sees them as cold and apathetic.
She manages to learn air bending reall fast, she never masters it.
Okay, who wan't to write about how badly this missunderstands the lore of Avatar first?
Lily: The kicker is, in order to be the best kind of sibling, you need to be the kind of sibling your sibling wants. And if they don't want somebody who coddles them, then don't be somebody who coddles them.
Whereas a crappy sibling would do nothing, or take the most agressive option automatically without finding out what they sibling wants. Always offer to support someone but do it on their terms.
A conflict in her Nivatar story would be that Lavi get's in a relationship in each nation they travel too (they spend 3 years in each nation), whith whom she has to break up each time the travel to a new Nation. Lavi can't stay because she has to travel with Niva, and the girlfriends can't come with them because ''they have lives they can't leave behind''. The fourth GF, the one that hates Niva, notices this pattern and tries to convince Lavi to ditch Niva to stay with her, which results in a fight. Niva gets revealed as the Avatar to Lavi's GF and the GF breaks down realising that she ruined everything.
Lily: Niva then does something that Niva pre-journey, would never have done. She actually call out her sister about how she treats women in her life.
Telling her that is unfair of her to get into these serious long term relationships when she has no intend of staying. Traveling with Niva means she is constantly on the move, which means can't be laying roots because the roots are gonna be ripped out again. It is not fair to the women whith which feeling she is playing with.
This is, to me at least, the best way to demonstrate Niva's growth. Her unhealthy attachement to her sister is the thing she spends the whole series moving past.
Before her journey, Niva idolized Lavi, she thought she was the greatest woman alive. Now Niva is activelly criticising Lavi's behaviour, which is honestly quite callous.
It is also something I would imagine the audience would struggle with as well, because she's speaking in the defence of someone who just tried to kill her. In fandom, as well as in online discourse in generall, people tend to be quite immoral if a character is presented as a bad person. The only simpathy they get, is platitudes for the other characters' ego.
Oh my god. Where to even begin.
There goes Lily's defense of bloodbending again:
Lily: Bloodbending is one that suits Niva's personality the most. Niva ultimately doens't like to cause conflict if she can help it, and she doesn't like using violence, so a tecnique that let's her imobilise seems fitting.
Given that blood bending has so far be only used by malicious people, the technique is still quite underdeveloped. And I feel like Niva would try and define the technique to do things more than just control and imobilise. I can already think of a myriad of uses with the ability to manipulate or control the body has, that are benefitial or at least benign.
Please tell which those would be Lily. I'd LOVE to hear it.
Lily: The obvious reason that blood bending as a technique is forbidden, is not because it is evil, but because it's an insta win condition, it completely destroys all tension in a fight. So making it forbidden means the heroes can't insta win every fight. But the obvious answer to hat question for Niva is that because it is a forbidden technique, it is a bad idea to be seen using it. Hence why as the Avatar, she wouldn't use it all the time.
Lily also has an idea for the Avatar state. Niva is unable to master it due to being unable to open her ''thought chakra'' cause that would require her to let go of Lavi.
Lily: The avatar state only come sout if she's in danger scared, or angry. When the avatar state come out like this, the avatar seems to be completely out of control and acting on baser instincts. In ATLA the avatar state often brings out the very worst in Aang. I feel that for a firm grasph on a characters positive development, you need to know what their failure state is. What is the worst possible versions of themselves, that they constantly need to be moving away from. Niva's entire character arc is about independence and breaking the enmeshment with Lavi and developing a hgealthier and more equal relationship with her. She needs to do this in order to be a good Avatar, in order to be kind and caring for other people and in order to take her responsibility seriously.
Her ''failure state'' the is someone who rejects all those things. (praphrased from here on out, because her monologe was too long) Where she resents and rejects being the avatar, takes Lavi under her armpit, leaves and lets the world fend for itself. That's who she is when she is in the avatar state.
She ''stole'' this idea from Spider-Man 3 goth Peter dance scene. The idea that when Peter wears the black suit he is the cringiest motherfucker alive.
Lily: And that's bevause the black suit is bad. You should hate it when he uses it.
That's about as far as I can go today.