Re: Autistic(?) nephew - should I tell?
I don't think this is something that can be left as like you said early intervention is so important.It's a very difficult thing to except and better coming from a supportive family member than to wait for him to be flagged up once he reaches school age. You could suggest they do the MChat online checklist and that way his difficulties may become more aparent, after all if they feel he is developing normally then he will simply pass the test and it will show he is unlikely to have autism, or that he is at risk.
You could give them some factsheets regarding the signs of autism. Its unlikely they would ignore them. It may be they just don't want to except there are difficulties or want to sit and wait to see how he progresses, however whether they deal with this now or in the future at some point this is going to have to be adressed, and it is going to be much better for your nephew if he recieves this support now.
Does he attend any early years settings, if so have they mentioned anything. Do your inlaws have contact with children of a similar age to compare?
My son is 3 and recently diagnosed with autism, he presented similar to your nephew, poor speech,flapping, toe walking, jumping, sensory difficulties, poor eye contact, difficult to engage, frequent rages,not responding to name,lining up toys,constantly opening and closing things, food aversions the list goes on.
I hope you can convince your inlaws to take him for an evaluation, after all it can only help your nephew, if nothing comes from it then they can say I told you so, however if you are right then your nephew will have far more potential for his future development and learning.
Hope this may have helped and feel free to write back if you have any questions.