It is none of your business.
I imagine that you love your cousin & her son, but this is their business and the best role for you to play is to be supportive & provide whatever they need, when asked.
If she asked you or expressed some concern to you about his development, I would definitely say to her, "well, I've read this".....and proceed to give her information. If she doesn't bring it up, I would do nothing.
I don't think any of us like another person...