Autism spectrum is very wide and we are all on it somewhere!
I wholeheartedly agree with that one!!!!!!!

I think its really difficult, because there are so many aspects to autism which many non-autistic kids display as well.

But I do just want to say, as an ABA therapist (working with kids with ASDs) and my bro has CP, if you SIL isn't ready to hear that her son might have special needs, it doesn't matter if he is assessed or diagnosed, chances are she won't be ready to have him in any form of early intervention. Not because she is a bad mother, but because accepting things like this takes time... I would say she is aware of any difficulties or any milestones her son has missed, and is dealing with this. Even though early intervention is so important if this little one does has ASD, if she isn't ready to even hear that he son might have this, chances are she isn't ready to start any kind of intervention at all anyway.

Maybe instead of trying to convince her to get her DS assessed, maybe a better approach would be to chat about what it would mean if her son did have some kind of special needs...she might have her own issues, guilt or blame about "how did I let it happen" or something similar (even though its not her fault if he does, that kind of thinking is almost inevitable).

Remember to be gentle with her... if you are noticing something, chances are she has noticed as well, and it is probably hitting her hard.