Sign up for the Slatest to get the most insightful analysis, criticism, and advice out there, delivered to your inbox daily."> Sign up for the Slatest to get the most insightful analysis, criticism, and advice out there, delivered to your inbox daily. Dear Prudence is Slate’s advice column. Submit questions here. Dear Prudence, I have twin girls who are 10. Their best friend, “Sarah,” is 9 and lived across the street until a few months ago, when her parents divorced. Sarah moved with her mom across town while her dad moved his girlfriend in with her two girls. It is a...