I'm looking for this episode

a sergeant and a constable happen upon an apparent couple while searching for a kidnapped child. Turns out that the nervous couple have nothing to do with the missing girl, but are skittish for an entirely different reason. The constable is disgusted and wants to bring the two in, but the sergeant points out that they should be left in peace, as the siblings are miserable enough as they are- hiding from the world and taking care of their senile mother, who berates them and their relationship every chance she gets - and they're not doing any harm to anyone else.