The Spencer Churchills (dukes of Marlborough) are descended from the first duke through a daughter, who had married the earl of Sunderland, family name of Spencer. So Sunderland became a secondary title of the Marlboroughs, while their cousins were elevated to Earls Spencer.
Since then (the early 1700s) there has been other intermarriage, like most of the grand aristocracy. No doubt the tiara came into the Spencer family through marriage, though purchase was also a possibility. The Marlboroughs were quite chattel-rich but cash poor before Miss Vanderbilt came on the scene.
Wikipedia is a surprisingly good source for some of this stuff.