Complex sentence: Li Shimin's grandmother Duchess Dugu was a sister of Empress Dugu, both of whom were daughters of Dugu Xin (獨孤信), a major general during Sui's predecessor dynasty Northern Zhou.
Split into simpler sentences: Li Shimin's grandmother Duchess Dugu was a sister of Empress Dugu, both of whom were daughters of Dugu Xin (獨孤信). The latter was a major general during Sui's predecessor dynasty Northern Zhou.