If you do manage to get your brother to listen to yur concerns, tell him to ask the hv to perform the CHAT test. It is a screening test which flags concerns for ASD, and your niece is the right age for this.
You could google it (sorry, am on iPod so can't do links) and print it out and show your brother maybe. It is a simple series of questions which highlight normal development, and it sounds from what you have described here, as though your neice would not pass it.
My dd1 was much like you describe at 18 months. She is mow 6, has severe ASD, and is a happy, cheeky, sunny little girl. She is still delaye in her development, and is more like a 3 year old in many ways, but she is doing well.
The weight thing could be a concern, but you say that healthcare professionals are not worried. My dd2 was only 14lb at 16 months - she was truly tiny! Mind you, she is only about 27lb now, and she is 4... She had a lot of tests as a baby, but the conclusion is that she is just petite.
You sound like a lovely caring aunt - does your brother have any other children, and are he/SIL up to speed with what a child should be doing by this age?
It must be so difficult for you - good luck 