I work with special needs children adn this question is very difficult to answer as its more a collection of things and it can be different for each person. It is also different for girls as apposed to boys and best to seek a professional opinion.

The best advise is below. See a health professional if you have concerns. There are a range of tests etc that they do along with watching how they behave/interact at school. Speak to someone about your concerns.

I would speak to a professional.

There are many signs of the spectrum we all can have, so it’s important to be careful with pulling conclusions. Also young children all show some of the signs in regards to short attention spam for example or response on strangers, which is however just a developmental stage. I agree it’s best first to carefully observe and chat with the parents. They are the ones who have to come to a conclusion themselves and decide if and when they want seek professional help.

Girls can have less obvious/stereotypical symptoms than males as girls often get a lot of socialising so are better at hiding or internalising some things. Everyone is different – some kids are in general more sensitive without being on the autism spectrum but sensitivity to textures, noise, change etc can be one sign.

I heard from two years old, you can see a backwards step in what they already know

I would not jump to conclusions, you have not stated age so hard to say whether it is just a stage they are going through, but you need to talk to the parents and discuss it without accusations or too much worry for them.

Many signs, speak to your maternal nurse who can run a few questions. Raising children site is fantastic

There are many signs which MAY mean that a child is on the spectrum. But a child can have signs without being on the spectrum too. If you’re honestly concerned, you would be better off talking to your nieces parents and they can make the choices they would need to make from there. In the end it is their decision whether they take their daughter to a paediatric for assessment and possible diagnosis.