The girls just started sleeping at friends' houses. They've spent the night with their cousins since they were infants. They are completely comfortable spending the night with any relative. They just started spending the night at their friends' houses and so far, so good.
We like this development. Whenever the girls spend the night somewhere, we have a night to ourselves. Sometimes we go out to dinner or to see a movie. Sometimes we just stay home and watch TV in our qiuet house.
Of course, we do reciprocate, but it's not the same for those parents. When their friends spend the night, their parents still have one or more children at home. When the girls spend the night, we have a quiet house.
This is one of those times when having twins is an advantage over have two children of different ages. It's an advantage we're enjoying greatly.