Uptodate, its very difficult when you are a gran and an MIL into the bargain and you instinctively feel that something is not quite right.
If it was something obviously physical it would be much easier to mention to your DI, that's why it is a minefield.
Given that that you don't have a good relationship with your DIL, I wouldn't say anything for now, but if you get to a point where you feel you have to, think carefully how you will say it
If you are in the UK, your DIL's health visitor be checking milestones and any concerns will be noted, even more so when your DGD goes to nursery.
As a teacher, who worked with children with autism, many already had a diagnosis, prior to coming to school and support was already in place.
However, no two children develop at the same time, the milestones are there as guidance.
Nezumi has offered some very sensible and measured advice, based on personal experience, so it might be worthwhile to take it on board.
As often happens with threads like this, you will see posts offering all sorts of opinions and advice, some helpful and some not so helpful. Take time to mull over them, before making a decision. I wish you all the best.