Complex sentence: In order to marry her, Gian Gastone would have to convince his father to allot him an allowance of adequate stature; the Grand Duke refused, and both the cardinalate and the Portugese marriage never materialised.
Split into simpler sentences: In order to marry her, Gian Gastone would have to convince his father to allot him an allowance of adequate stature. However, the Grand Duke refused, and both the cardinalate and the Portugese marriage never materialised.