From the kids perspective, their sibling is also their cousin.
I have a genetic half brother in my mom’s side. But since he’s older than me, my biological father adopted him, and he has literally always lived with only us, I just view him simply as my brother. I always have.
My mom has an identical twin. Her twin’s children - my cousins - are genetically just as related to me as my half brother, who I view simply as my brother.