OP I think it’s unimportant compared to other potential effects on his development. If your daughter in law was so lazy she couldn’t be bothered with him, put him in front of the TV all day and didn’t engage with him I’d be far more worried than if she left his potty training late.
What time do you get up and go to bed?


My DGC was part-trained from 2 yrs but just couldn't be bothered - lots more interesting things to do! - so went to nursery in nappies. At 3 1/2 yrs, I guess felt different from little friends, decided to give up nappies and use the toilet. All very relaxed and much more so than the constant trekking to the potty/toilet with younger ones. I have to be honest, I didn't approve at the time but it worked out fine. I'm learning to keep quiet!