How important is it to diagnose a child suspected to be on the spectrum?

Hello everyone, I’d like some advice regarding my niece. 

She is seven. My sister and I suspect that she might be on the spectrum due to some issues in her behavior. She gets very confused and overwhelmed when meeting anyone, including children considerably younger than her. She’s been in nursery then school since she was 3, but has not made a single friend to date. She does not respond to her name being called sometimes, she refuses to talk sometimes, she prefers to play alone rather than with her 5 year old sister, and recently her teacher complained about her sitting under the table in class rather than on her chair. When asked why she remains silent  and doesn’t give an answer. There are other things that we noticed.

Our main problem is with our brother, her father. To him she is perfect and there is nothing wrong with her and we are interfering in the child’s life. 

Now he is her father and we have no say in it, but we love her and worry, so what I need to know is: how important is it for a child like her to be diagnosed? Will it make a huge difference? Will she still be OK without being diagnosed? Considering the hiding under the table for most of the class time, and an incident on the bus only a week ago when she turned red, got very confused, look at her feet and pushed her body as far back as she could because a child (my guess is he was 2) smiled at her and waved his hand, considering this, can she manage without help?

Is it worth interfering in my brother’s decisions or should I just leave them alone?

I appreciate any input on this, especially with regards to the girl’s well being that is the main issue here.