What does it mean if a girl says you the you're like a wife to her? Or if a guy says you that you're like one of his bros?

I seen a few girls telling this to some guys they're going to marry. And I seen a few guys saying this to their partners. I don't see what's the issue with them. Why can't they use girly or male pronouns without saying these senseless words to each other? What they want to convey to the other person by telling them these?

What does it mean if a girl says you the you're like a wife to her? Or if a guy says you that you're like one of his bros?
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