> First question, Is 'Rumor has it' an idiom, therefore "Rumors have it" is incorrect?
Yes.
>I find "the three children that his brother and his ex share." confusing.
Perfectly clear to me.
Because
he's going to marry the woman who was the wife of his own brother, she
convinced him to fight over the custody of the three children that she
has procreated with his brother, because they are hers too. She is not
interested in the other two, to whom she is not a birth mother.