DEAR MISS MANNERS: Every year, my family spends a week at the beach with my in-laws. They pay for the beach house, which is something we can’t afford and for which I am always properly grateful. At the in-laws’ suggestion, my son and daughter each bring a friend on these trips. Last year, they rented a house with only enough bedrooms for two of the four children to have a room. My son and his friend shared a room, and my daughter and her friend had to sleep in the hallway. It was no big deal; there’s not much difference in their ages, and there’s no reason why one would have more or less of a …