Did Glenn Miller's wife ever marry again?

By all accounts yes, for most of his marriage to Helen Burger. He spent as much time as he could with her and doted on his first adopted child. (The second was adopted after he was serving in Europe.)

Recently there have been rumors - and I emphasize that they are nothing but that - that the pressures of wartime service led him to an affair with a woman in Paris during 1944. Even so he remained devoted to Helen. When his health began to fail in late '44 he wrote to his brother Herb to make sure Helen and the children would be well-cared for if he should not return to the U.S. As we know now, the plane crash that took his life in December 1944 made those preparations absolutely necessary.