Your child's reaction to a sibling getting sick enough to go to the hospital is not really atypical. Even neurotypical kids will sometimes respond that way to the stress of a family member having an urgent illness. Actually, so will some adults.

That your child is diagnosed with anxiety, ADHD, and OCD makes this even less surprising.

Keep in mind you are also experience stress and anxiety due to your other child's illness, compounded by these challenges with your 15 yo. So your reactions may also not be ideal.

I would view your 15 yo's behavior not as "selfish" or even lacking in self awareness, but as evidence of poor coping skills. Assume he's experiencing a spike in anxiety due to the sibling's illness, driven both by fear for his sibling and also a [very normal!] fear of abandonment as he's watched your focus shift to his sibling. Then think about what healthy coping skills for this would look like, and see if you can coach or model these.

For my SN kid, one of the best ways we've found to help her deal with high stress family situations is to take some time to talk it through with no judgment. Even just 5 minutes to acknowledge that a situation is scary, stressful, or is resulting in her having unmet needs (attention, sleep, a reliable schedule, etc.). I listen to her complaints and validate her feelings. Often I will share that I feel similarly, that the family stress is also making me feel anxious or frustrated. Then we talk about ways we can get at least some of those needs met while we get through the family crisis. This process helps teach valuable coping skills and with practice, my DD is now able to initiate these conversations herself without having the tantrum first. It has helped her become more aware of how she responds to stress and that her grouchiness and irritability is often caused by suppressed feelings like fear.

Agree with PP that he's not a bad kid. Your family is dealing with something challenging and he lacks coping skills and has some built-in obstacles to helping him deal with stress.

I've had similar experience when my attention is diverted to a higher priority issue for another loved one, leaving SN kiddo without ordinary levels of attention or routines. For your DS, the situation was probably deeply unsettling, and maybe not just because he didn't get his regular dinner, etc. He likely doesn't know how to process the unknowns about why the health issue happened to his sister, whether it will happen again, whether it could happen to him or to you, etc. All pretty scary stuff for a concrete thinker, but it probably came out sounding like a jerk. My hunch is he was bewildered. Toughest part of SN parenting is the patience required when you're stretched too thin. If you unloaded on him, let him know you were just tapped out, and from your perspective, it sounded like he wanted you to prioritize something small over something more important, like his sister's health. Sometimes that's just impossible when exceptions arise, and we all need techniques to go with the flow when frustrating or uncertain situations arise-- that might be super hard for your son. Rationally he probably will understand. I talk to my SN kid about what to do when expectations are unfulfilled due to circumstances beyond anyone's control. Sounds like a bad night, and I'm sorry. He's not a bad kid. I think just clueless and scared and doesn't know how to process everything-- it wigged him.

I am so frustrated with my SN teen

Borderline ASD
ADHD
OCD
Anxiety
Language disorder

Refuses therapy or medicine

Sister goes to hospital and sleeping in our bed after a night of vomiting and fever and he breaks down about HIM and change in his and our routine because of HER getting sick.

Selfish jerk or just not self aware? He’s clueless so I am calling him out for thinking only of him and he just doesn’t seem to even understand what I am saying or having a layer of care or concern for his sister vs disruption in his routine. So frustrating