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One of my major characters is between the ages of 10-13. She is supposed to be terminally ill and she's supposed to die at some point during the story (while still within that age range). The problem is, I'm struggling to find a terminal illness that would afflict someone this young that meet my necessary conditions:

1. It must begin having an effect on her from a very young age.


2. Is terminal and can kill her by her early teens (14 years old at the latest) even with access to the best medical resources. Not that people can't live beyond that age, but that medical resources can't guarantee a prolonged life.


3. It is NOT cancer.


4. Would leave her in the hospital frequently and for prolonged (business week-length minimum) stays.


5. Is NOT a dominant-allele genetic disorder. (If genetic, it needs to be recessive or not guaranteed to affect a person who has it. She has multiple sisters who should be healthy is why.)


6. Does not result in any visible outward deformities.

While I could just default and use cancer, I have personal reasons why I can't do that. While I could go with a generic nameless cough, (especially since I don't plan on name-dropping the disease,) I'd rather use something specific to base my character's actions and interactions around.

My question is: What diseases are terminal that can affect/kill girls of this age range and start having an effect earlier on in their lives without being guaranteed that their sisters will have it too?

World background: Real world, modern day, without magic, without sci-fi tech.

If there are other things I need to list for this to be on-topic, please let me know and I will edit to reflect that.

Congenital heart disease.

https://www.marchofdimes.org/baby/congenital-heart-defects.aspx

Congenital means present at birth. Congenital heart defects are heart
conditions that a baby’s born with. These conditions can affect the
heart’s shape or how it works, or both.

Congenital heart defects are the most common types of birth defects.
Birth defects are health conditions that a baby’s born with that
change the shape or function of one or more parts of the body. They
can cause problems in overall health, how the body develops, or in how
the body works.

Congenital heart disease lends itself to a story because the children are cognitively normal, not contagious, not doomed like someone with cancer but limited for cardiopulmonary reasons. They can be maintained with surgery and medical care but many die young - in childhood or young adulthood. Some can be cured with a heart transplant but even if that is a possibility for your character, transplant recipients still have issues and of course can also have issues from the immunosuppression required to keep the transplant.

If you leave it at that - girl tends to be blue, out of breath, sickly, born that way - that would suffice for a story. If you need to get into the weeds read more on Tetrology of Fallot. It is one of several congenital cardiac syndromes that could work for you.

Cystic fibrosis

Despite the name, the problem there is you produce much thicker body fluids than normal. The largest problem is mucus, which can coat the linings of the lungs, thus causing problems like lung infections, which often require long hospital stays to fix. It's often caught in infanthood, but there's no way to treat the underlying problem, so you treat the symptoms. Some cases do manifest outwardly (and subtly), but many do not.

As to it being an inheritable trait, it's quite possible that she got unlucky in the gene pool

Every person has two copies of the cystic fibrosis transmembrane conductance regulator (CFTR) gene. A person must inherit two copies of the CFTR gene that contain mutations -- one copy from each parent -- to have cystic fibrosis.

Her sisters are healthy because they got one good gene (they are still carriers of the bad one, though).

This fits your story well, because she could seem to be holding up fairly well, and then abruptly take a turn for the worse and die from a lung infection.

You could go with a variant of the Rett Complex syndromes. It is a rare (1:10000) genetic condition which is usually acquired "de novo" (i.e. with both parents healthy), only fully understood twenty years ago, which mainly affects females (males die shortly after birth except in rare cases).

This link describes the most common and severe forms of RCS.

Some milder forms allow normal development up to a certain age (6-10 years), but have secondary cardiac involvement. As a result, the victim may experience fainting and/or seizures (which would lead to her condition being discovered), or sudden death due to cardiac arrest (triggered by physical effort, emotional stress, or idiopathic). The heart itself is healthy, so transplant is not an option.

Due to the variability of the disease, it could be plausible for a victim to show few symptoms (hand wringing, slight unsteadiness when walking, mild speech impairment, restlessness) and need nothing else except for a non-periodical SSRI course (anywhere up to two weeks' hospitalization and home follow-up).

Symptoms are progressive, and a mild form might look to a life expectancy in the forties; there would be no reason to be certain of death before age 14 (unless the form was more severe, but in that case you'd be looking at heavier development anomalies - microcephaly, etc.).

She got AIDS from her mother during her gestation. Her mother got infected after giving birth to her older sister, so the sister is not infected. Or alternatively she could have got it via a transfusion in very young age.

Current medications of AIDS can improve life expectation, symptoms start showing from early period, however there is no cure yet.

Polio comes to mind.

I am pretty sure about points 1, 3, 4, 5

Regarding point 2: People can survive Polio, yet I guess this depends on the available treatment.

Regarding point 6: while deformations have been observed, complications vary for individuals

1. It must begin having an effect on her from a very young age.


2. Is terminal and can kill her by her early teens (14 years old at the
   latest) even with access to the best medical resources. Not that
   people can't live beyond that age, but that medical resources can't
   guarantee a prolonged life.


3. It is NOT cancer.


4. Would leave her in the
   hospital frequently and for prolonged (business week-length minimum)
   stays.


5. Is NOT a dominant-allele genetic disorder. (If genetic, it
   needs to be recessive or not guaranteed to affect a person who has it.
   She has multiple sisters who should be healthy is why.)


6. Does not result in any visible outward deformities.

On a historical note, tuberculosis used to kill a broad range of ages, and can take years to kill. In this case, you need only specify that this is a new strain, and one which is resistant to all current antibiotics. Which is not at all a far-fetched assertion, since strains exist which are, in fact, resistant to the major antibiotics. Google on "drug resistant tuberculosis".

Kidney failure

This could be from ingesting some toxic substance, or through some developmental abnormality. There are inherited conditions which would cause this as well, but there are plenty of other possibilities.

Dialysis is required for any kind of medium-term survival. However it does not work forever. Eventually toxins will build up to damaging levels, and the patient dies. Or the patient develops sepsis or some other hospital-related bug and dies.

A kidney transplant will solve the problem. However there are many more patients needing transplants than there are matched donor organs. Sadly, many patients do die whilst waiting for a suitable donor organ.