Complex sentence: The origins of this church are related to Franciscan convent existing in the Trastevere quarter of Rome since the 10th or the 12th centuries, which had an annexed church dedicated to San Biagio (St. Blaise).
Split into simpler sentences: The origins of this church are related to Franciscan convent existing in the Trastevere quarter of Rome since the 10th or the 12th centuries. This had an annexed church dedicated to San Biagio (Blaise).