Offer advice and suggestions when asked. And only when asked.

Otherwise MYOB.

The sleeping til noon sounds weirder than the speech.
I have two boys who did not speak at all. The oldest did EI for speech and the youngest did not. Both started talking right before 3 and only sparingly. They're both in elementary now and completely normal.

Leave them alone. Looking back, we pushed for speech therapy that young and while our child had significant long term speech issues I don't think the ST did much of anything. Waiting till age two makes more sense.

There are kid events and kid socializing that happens in the evening, OP. I have several friends whose kids take long naps and are up until 9 or later. It didn’t work for me — I’m a morning person and my kids want to be out of the house by like 7 — but I fully understand that it’s fine. No or limited talking at 18 months isn’t a big delay. Both my kids were late talkers and frankly I didn’t have the energy to fight with EI when I didn’t think it was going to be necessary and it wasn’t. I think you’re projecting your experience with both your children’s sleep schedules and special needs onto your niece and need to step back a bit.

This really strikes a nerve with me. My parents disapprove of many of my brother and SIL’s parenting choices but don’t dare say anything directly so they try to put me in the middle. I hate it. Don’t be that person, OP.