My nephew has lived with us since he was born and I have two daughters who live here as well. They are all in thier teens. They have never worn hijab infront of him but recently someone brought this to my attention. I am wondering what should we do about it?

Does the non-marham rule apply to 1st cousins? Or does the rule still apply even though they have been living this way with eachother their whole lives.

They are na mahram to each other and you can gently explain to your daughters to observe hijab in front of their cousin.