Before Edward, Prince of Wales famously abdicated his throne for American divorcee Wallis Simpson, he loved another American woman: Thelma Morgan Furness, sister to the first Gloria Vanderbilt. This is her story.   The daughters of an American diplomat, Thelma and Gloria Morgan were stars of New York social scene in the early 1920s, dubbed “the magnificent Morgans.” Both would marry into wealth and privilege beyond their imaginations, Gloria to Reggie Vanderbilt, and Thelma to a viscount. Thelma begins an affair with Edward, the dashing Prince of Wales, that will last nearly five years.   Then, in 1934, Thelma s life is upended by her sister Gloria s custody trial — a headline-grabbing drama known as The Matter of Vanderbilt , which dominates global news for months and raises the bar for tabloid sensationalism. Back in New York, sued by members of her late husband s family on charges of negligence, unfit parenting and homosexuality, Glor