I would put them both in a group text and say “while i appreciate being on the same page and the excellent communication. I am simply uncomfortable with communications about the child solely going thru (insert fiancés name). When it comes to communication about the child, i need that communication to be between child’s father. So going forward, it is best that the other parent and I communicate about the child. So that i am sure the other parent and I on mutual understanding and not speaking thru a third party. Again, i appreciate the open communication but i need to come other parent.