Complex sentence: Rousseau would remain lifelong friends with Mably and his family, both Mably and his brother Condillac visited Rousseau when he moved to Montmorency, Val-d'Oise.
Split into simpler sentences: Rousseau would remain lifelong friends with Mably and his family. Both Mably and his brother Condillac visited Rousseau when he moved to Montmorency, Val-d'Oise.