Complex sentence: The second link to the Beauforts was between Buckingham's second son, Sir Henry Stafford (c.1425--1471), who was to be the third husband of Lady Margaret Beaufort, daughter of John Beaufort, 1st Duke of Somerset, and Margaret Beauchamp.
Split into simpler sentences: The second link to the Beauforts was between Buckingham's second son, Sir Henry Stafford (c.1425--1471). Third husband of Lady Margaret Beaufort, daughter of John Beaufort, 1st Duke of Somerset, and Margaret Beauchamp.