Need serious help understanding

My little sister is 3 years old, my parents and boyfriend seem to think she is autistic. There has been no official diagnosis because everyone my parents have asked for help have done nothing, even psychoiatrists amd other specialists. I believe its because her and my other 8 year old sister are spoiled brats. She was never an easy child to take care of, for three years its been nothing but screaming and crying but it is only getting worse as she gets older. She screams every morning withiut fail, screams when my other sister pisses her off, screams when she doesnt get what she wants, screams when she wakes up from naps and she could scream for hours on end. I have realised lately that she likes her little routine of going to pre-school for 4 hours then coming home and watching Youtube on the laptop. I have had serious arguments with my parents over a three year old using a laptop. It is getting to the point where my family is falling apart over this one child. I dont understand how they could see her as autistic I just see her as a spoiled brat. But I dont know. Has anyone else had to deal with this and how?