The moral is deeper than what it first seems...
What they are trying to say (in a metaphorical context) is "know your boundaries" and if guests are not respectful of your home you should have no qualms about asking them to leave. She was trying to be nice at first and gave them the benefit of the doubt but she soon learned that if you give them an inch they'll take a mile. This teaches us, the audience, not to make the same mistake and to speak up when you are uncomfortable around people who don't respect your space and your house rules. Very powerful movie.
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