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[Rumor has/Rumors have] it that Mike is going to marry his brother's ex-wife and fight for custody of the three children that his brother and his ex share.

First question, Is 'Rumor has it' an idiom, therefore "Rumors have it" is incorrect?

I find "the three children that his brother and his ex share." confusing. Could you please rephase it? Note that the brother has five kids, of whom 3 are from his marriage to his ex. Mike and his future wife are only interested in the three kids.

Thanks
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> 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.
Comments  
I was confused because the two 'his' refer to two different individuals, first Mike, later Mike's brother without any clue but logical thinking. Is this common in writing or the writer would avoid such contructions?

Is the following considered better?

the three children that his brother and his future wife share