From what I gather, Krystal “dislikes” Miles only on principle. She doesn’t get along with her brother, so (of course) she can’t be friends with her brother’s friend. Especially at that age.
I’m fairly sure that she could be friends with Miles if she had met him before Kyle did.
Speaking from experience, your mother is not wrong. Cloth burp rags make a decent shield, though. (Heavier than a wet wipe.)