There's really nothing you can do if someone has their mind made up. I wish I had the power to change minds... That would be awesome... But then again, it's probably a good thing if I can't control other people. The best you can I guess is pray and hope the mother will be more open to listening at some point.
Oner thing to hope for is that someone in the school system may make a refferal when he starts school.
If the mother is autistic or aspergers she may have concerns about whether her ability to cope might be questioned. Or she may have difficulty predicting outcomes and may have incorrectly predicted the outcome of going along the route you suggested. If the child is how the mother was as a child she may really not see there is any problem. I am not autistic, but my son is so similar to me and other family members (especially sensory stuff), that I really couldn't see what was different with him until I saw him in social situations.
You could send an apology note saying you are sorry about how they are feeling and that you in no way were trying to take over or tell them what they must do, but say as a 'professional' you felt you had to say something and that the door is still open (from your point of view). Then you will just have to leave it and see how they respond. As previously said, if the child has these problems by nursery/pre-school they should get picked up there.
You have been helpful in giving me another perspective. The mother is very non communicative and withdrawn. The child never sees or plays with other children. They
feel that we are judging them but also call us and ask for money when they have self
distructed.....after passing along several thousands of dollars...we feel that if they are unwilling to uphold their part...such as better communication with us and getting
their daughter assessed why would we continue to send them $$$$$. My husband is retired and I am semi retired so the $$$ is not endless. My stepson becomes very hostile when he is even asked a simple question about his wife or daughter and never brings them up in conversation. My internal senses are screaming concern about this situation. I have considered a CPS referral...would that be wrong???
I am from the UK so I don't know what a CPS referral is. I presume it is somelike contacting Social Services and telling them of your concern? I think it might put your mind at rest to talk to someone about this situation, without mentioning names/addresses etc, as I think the situation would be 10 tens worse if someone turned up on your stepson's doorstep. They would know it was you. But talking to someone might give you a better idea of who/how this child might get picked up by the system.
The money problem I think could be making the situation worse. They are not taking responsibility for what they are doing, and by taking money off you it also, to some extent, gives you more right to comment on their family life, which is not necessarily a good thing either. Maybe your husband should talk to his son about this.
I would just bide your time for now. Even if the mother is autistic/aspergers, it sounds like she doesn't have a diagnosis herself. It may take along time for her to consider that both her and her daughter need a diagnosis. It may take time for her to see that her daughter is not progressing as her peers are. Although early intervention is the ideal, it usually doesn't happen that way, especially if the child is quite able. Many children are diagnosed later, some only when they are adults. I found this article in Wired very insightful, because it says that being autistic does not mean you are less intelligent (something I have been saying for years). There will be plenty of time for 'catch-up', if that is the way it happens.
This is the Wired article. http://stag.wired.com/medtech/health/magazine/16-03/ff_autism