Mental issue. Doctor should i consult?

My younger sister is very different from her age girl she is 27 years old but her behaviour is look like less than 10 years old kid. She is very aggressive, no common sense, she cant take care of herself like daily routine work. Her hands shake lot when she do anything. She dont understand anything like general talk and dont know how to talk anyone. She is very much quit person. She love history and collects pens too much. I guess she has some mental issue. And i want to help her any how for her better future, i cant she her like this. My father is no more and brother dont like her and she also hates him. Plz help me what should i do, which doctor should i consult? Can she be treated by medication or something else. Plz plz help me.