I thought either a wedding planner or marriage councellor, someone who’s supposed to help you in your relationship. And obviously family members.
Feel free to mention personal experiences.
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.