Heiress and furniture designer Alexandra von Furstenberg (nee Miller) is to marry her boyfriend, Dax Miller, after a two-year courtship. Alexandra, the daughter-in-law of Diane von Furstenberg after previously being married to Furstenberg’s son Alexander, broke the news of their engagement to friends last weekend. Californian Dax, who runs his own interior design firm, and his bride-to-be were traveling in China earlier this month with fashion designer Diane, who was opening a retrospective exhibition of her work, “Journey of a Dress.” The couple later headed to Shanghai and the Thai island of Phuket, where Dax popped the question on the beach. Alexandra and Dax have been dating since 2009, but got off to a rocky start because her family was against the relationship, fearing Dax, a former events producer, would be a bad influence. But a source tells us, “He has been wonderful for her and won everybody over. They are very happy.”