LONDON — Johnny Hallyday, known as the French Elvis, built his six-decade show business career on old-fashioned rock ’n’ roll, but he also kept up with the times technologically. From 2012, his Instagram account shared a canny mixture of the personal and the professional, promoting his tours, his albums and his image as one of France’s most enduring stars.

And on Tuesday, it helped two of his children defeat his widow in the first stage of a legal battle over an inheritance that the French news media values at tens of millions of dollars.

When Mr. Hallyday died in 2017, two testaments were found in a safe deposit box. One of the documents, written in Los Angeles, appointed his wife, Laeticia, as sole heir and manager of his estate.

That will entirely excluded Mr. Hallyday’s grown children from two previous relationships, David Hallyday and Laura Smet, something not permitted under French law. The two have been fighting to prove that their father lived mostly in France and not in the United States.