The 5 year break allows her to see him in a different light now that he's grown up, had they kept being neighbors it may have been harder for her to see him in this fashion.
I think even though she thought of him as a brother, that declaration in the backyard stuck with her(and she probably talked about with Alexis, before and after the MC coming back into town, so Alexis knows).
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They talked about things since Erynn also knows about the stolen kiss.
Melissa knows even if Erynn never told her anything, simply mother intuition, she keeps pushing and teasing for them to get together, so she believes this is what her daughter wants, even if she doesn't vocalize it.
Ethan disliked the MC since he was a little boy, saw him as constantly getting in the way of his plans of conquering Erynn... and probably knows that the adult MC presents twice the problem.