Is it normal to have nightmares about my sister

She is like 25 and acts like a spoiled 5 year old to my parents. Not exaggerating that either- she's never had a job, failed out of college, all she does is stay at home playing on the computer and bitching at my mom. She has made my mom cry several times, she has no understanding of the world, she yells at my mom over the stupidest shit while eating food that my mom made. It's retarded, literally- I don't know if she is autistic or learning disabled or whatever. And before people get butthurt over that being offensive, I have mental disorders too, but at least I know that when someone does something for you, lets you live in their house, etc. you treat them with respect! I have trouble understanding the world too but that hasn't stopped me from getting a job, getting good grades in college etc, because I know it's what my parents would want. And I am the younger sister. I love my parents very much so it's very distressing that she treats them that way but idk what to do. I keep suggesting they have her mentally analyzed but they never want to believe anythings wrong.

So anyway, I have nightmares about her, the qualities are a little exaggerated but still believable, just her acting psycho and stuff to my parents, but it really bothers me.