Complex sentence: Henry Sinclair was the son and heir of William Sinclair, Lord of Roslin, and his wife Isobel of Strathearn, a daughter of Maol Ísa, Earl of Orkney.
Split into simpler sentences: Henry Sinclair was the son and heir of William Sinclair, Lord of Roslin, and his wife Isabella (Isobel) of Strathearn. She was a daughter of Maol Ísa, Earl of Orkney.