I am curious as to what you are seeing that makes you think Williams Syndrome (it is rare). Also, is your niece seeing a pediatrician? I ask, because at 4, most developmental delays for WS are fairly obvious, and I would be surprised that she has not received a referral.
Now that being said, I am going to specifically answer your questions about how to start the convo. I do not want you to take this the wrong way, but I am looking at this from the perspective of a pediatric nurse and the mom of a special needs child. No parent wants to hear that their child is delayed. Ever. Especially not from someone who is not a medical professional. So I just want you to be prepared that this convo will probably not be well received, esp if your mom is flying in just to confront the parents. Also, it is possible that they have a referral (and maybe even a diagnosis), but are not sharing that with other family members yet. My husband has a cousin who is an Aspie, his parents denied that their was anything wrong until he was a teenager. He had been receiving treatment the whole time, the parents just did not want anyone in the family to know about it. I was 17 when I met him the 1st time, and that was my first question, so it was very obvious.
If you are determined to have the convo, I would start with your observations about delays. Do not mention a dx. Something like, I noticed that ___ has problems with this task. Have you noticed the same thing? Then listen. You can also ask if their pediatrician is concerned. If it was me, I would prefer one of my family members to approach me one on one. If it was my in-laws, I would prefer they talk to my DH.