To say something or keep quiet, that is the question

While understanding your anxiety it really is too soon to make any judgement. It may be best to be led by the child, interact with her in the way thats suits her as the individual she is. I suspect all children are capable of not hearing an adult when they are engrossed in play or simply don’t want to interact. Please try not to worry. If you can try to step back from being overly concerned and enjoy the time you have with all the family.

Some children are just slower to talk. I have autistic children and they talked very early and at an advanced level. It's true a lot of girls aren't diagnosed till much later. What you describe could also be a hearing problem. If GD gets all her child health checks, I'd just watch and wait for now.

NO say nothing and don’t even presume it’s autism all babes meet milestones differently and she’s probably perfectly fine
Shes potty trained which a lot of parents don’t seem to manage at 2 years old these days, she understands all you say to her and is eating well
Enjoy her and let the parents do the same

I was all for 'saying something' until I saw your last comments.
It sounds like you don't have the best of relationships with your daughter in law, and don't think that your advice will 'go down well'.
I think that you've answered your own question there!
With the best will in the world, I don't know why you are asking here!
Unfortunately you have no option other than to say nothing unless you want to cause problems.

Keep your nose out of it. They're with their child far more than you are and probably are aware of the issues and have plans in place.