Wait when was she rude to Marinette? (Are you talking about Frozer? B/c in that case, wouldn't you also dislike it if somebody else was taking the attention of your date?) And there's nothing wrong with Kagami having feelings for Adrien; you can't be mad at her for that.
And Nathalie... it seems weird. If she loves (as implied by the show) Mr. Agreste, then why is she trying to help him get Emilie back? Unless that's out of love and wanting to make him happy...?