What Causes Intellectual Disability And Abnormal Growth And Development?

My sister is 15 years old and has the following symptoms: reads at the kindergarden level, has not started her menstrual cycle yet nor is developing any, she mispronounces many words, and cannot complete a grammatically correct sentence. She is currently enrolled in the special ed. program at her high school. I know she s not the same as everyone else, but i never knew her diagnosis exactly. What could it be?