Why does my 6 yrs old niece have to be carried out to the car to school?

Is it bullying or autism?  We got to the heart of the kids bullying her.  Typical bullying in a under privileged neighborhood.  My niece came to stay with us.  She has two siblings. 4 and 2.  One has eczema and cries throughout the night often. We figured she needed a more stable enviroment for awhile. My son is 16 and my mother lives with us, whom my niece is greatly attached.  She was fine for 4-5  days.  Then one morning threw a tantrum and would not go to school. She seldom looks us in the eye.  She lives in a dysfunctional home, but we thought she was getting better.  I even gave her a dollar a day to see who won the race to get ready in the morning.  One morning all hell broke loose.  No way was she going to school, even if the police came. She does not look you in the eye.  She does not speak clearly, but can tell u every planet in the universe and what happened bad at home.  Please help.  I don't want to seem like the bearer of bad news in the family.  She is beautiful and smart, but will not even go to school when the security guards carry her in.  At 6 years old.  Please help.  The doctor just diagnosed her 2 1/2 yr old sister with autism, I don't know how, in a 5 min eval, but I suspect autism in the six year old for years.  What can I do?  Her mom is 24 with three kids...and only cares when its convenient. Please tell me what I can do!

[Sally44]

The only way for a child to get a diagnosis of an ASD is for them to be assessed and observed.  Can you speak with the parent about asking the GP or school to assess her?  Alot of her tantrums and school refusal might be down to not being able to cope in school due to difficulties associated with an ASD.  For example she might struggle with the language side, either the understanding of what is expected of her in her lessons, or might struggle to use language herself and feel very isolated and frustrated.  Or she might struggle with social interaction or sensory issues.
If the school are having difficulties they should be involving outside agencies such as a Speech and Language Therapist and an Educational Psychologist to come into school and observe and assess.  
I don't know what country you are in, and therefore it is hard to say how exactly a child would get a diagnosis and support where you are.
If you take your niece to school you could make an appointment to see the headmistress.   The school probably don't know about the younger sibling with autism.  But there is some evidence of a genetic link and therefore if one child is on the spectrum there is a higher than average risk that others will be also.  
If you talk with school that might involve going behind the mother's back.  That makes it very difficult.  So I think it would be better to speak with the mother first.  
If that simply is not possible.  Then you could write a letter into school and go and give it to the headmistress at the same time that you have arranged a meeting with her.  
School are responsible for making sure a child's educational needs are being met.  And therefore if they do have reason to suspect that she has some difficulties, then the school are supposed to speak with the parents and bring in outside professionals.
Or you could visit your local GP and ask them for advice on what to do.