Complex sentence: On 12 May 1879, George married Duchess Therese Petrovna of Oldenburg, a daughter of Duke Peter Georgievich of Oldenburg and Princess Therese of Nassau-Weilburg.
Split into simpler sentences: On 12 May 1879, George married Duchess Therese Petrovna of Oldenburg. She was a daughter of Duke Peter Georgievich of Oldenburg and his wife Princess Therese of Nassau-Weilburg.