Complex sentence: Gan married his first wife, Koh Chwee Neo, at the age of eighteen and in 1859 adopted his first son, Gan Tiang Tock, who was an important partner in the building of his fortune.
Split into simpler sentences: Gan married his first wife, Koh Chwee Neo at the age of eighteen and in 1859 adopted his first son, Gan Tiang Tock. His first son he adopted was an important partner in the building of his fortune.