Complex sentence: Pedro was the first-born of 15 brothers of modest condition, started his studies in the first school founded on rebuilt Chillán, and soon continued them in the school founded by the Spaniard Jose Martinez.
Split into simpler sentences: Pedro was the first-born of 15 brothers of very poor background, and he started his studies on the first school opened in rebuilt Chillán. Soon afterwards, he continued them at the school founded by the Spaniard Jose Martinez.