Sometimes the Mishna the facts of the Mishna are more compelling and insightfull than the proposition they seek to demonstrate. On 86A, the Mishna gives the exampe of a man who leaves his home for Shabbat without contributing to the Eruv for the Chatzer (combined homes). The question is : does his failure to contribute restrict the usage and carrying of other in the Chatzer. The Mishna gives an answer, only if he goes to his daughter in law.
What a great Mishna, first what a great lesson that the Chatzer is dependent on all the residents. All people are critical and necessary. But the second piece is pricelss. Read the Mishna and Gemara to find out what the Mishna thinks of a daughter v. a daughter in law. You are not afraid of a son-in law, but you are afraid of a daughter in law. Read it.