You are a GREAT sister and aunt!!! :)
I don't have a suggestion for a vitamin since my kids are a little older but, since you sound very open to alternative medecine, (rather than just a drug for everything!), I do have a suggestion re: her eczema. Eczema is actually an inflammatory/autoimmune condition. It is a long story but gluten and dairy (and sometimes other food intolerances) are often the cause of eczema. I found out by talking to a nutritionst that I was gluten intolerant and removed it. My skin was better, my thyroid problem went away, I had MUCH more energy, my immune system improved, constipation went away, my muscle and joint aches disappeared, stomach problems disappeared, etc, etc, etc! I then removed it from my daughter's diet as well. Her long-standing eczema cleared up very quickly as did our asthma! We were able to go off all of our allergy and asthma meds. It was amazing.
Many people with sensory disorders, ADD/ADHD, Asperger Syndrome or Autism see an improvement in their behaviors and symptoms when they remove gluten and casein from their diets.
Gluten intolerance and celiac disease are genetic conditions and are really underdiagnosed. People suffer for years with increasing symptoms and most doctors are not quite getting it yet. They were never able to figure out why i was having so many problems. I encourage you to read about them online. Just search "gluten" or the word "celiac" along with "eczema" or any symptoms she or anyone else in your family is having. You will find lots of reading. If anyone in your family has autoimmune conditions (including diabetes), cancers, canker sores, digestive issues, migraines, unexplained aches, thyroid problems, numbness or tingling in hands or feet, Alzheimer's, fatigue, allergies or asthma, depression, anemia... You might want to look into this. It is more common than people think! :)
Sometimes, REMOVING things from the diet can be just as important as adding things. Food intolerances are different than allergies and won't be recognized on an allergy test.
A good nutritionist would be able to suggest a supplement for your niece. Let me know if you need anything else. I have lots of books and online articles I could share.