Welcome to thalpal,
From what you have posted, your niece is Thalassemia Intermedia and her Hemoglobin levels, spleen size need to be monitored. If the HB levels go too low, she would need transfusion.
It is also important to know the mutations as it will help in treating it, so if you could ask for getting her mutations tested it would be great.
Hydroxyurea or Butyrate therapy might be helpful in increasing the hemoglobin.