From Wikipedia:

The movie opens in 1917. Baby Jane Hudson (Julie Allred) is a vaudevillian child star. She performs to adoring crowds, and there’s even a “Baby Jane” doll. Jane is also a spoiled brat, and her doting stage father Ray (Dave Willock), gives in to her every whim. Her jealous sister Blanche (Gina Gillespie) watches from the wings.

The movie then jumps to 1935, and the sisters’ roles are now reversed. Both are movie stars, but Blanche is the successful and glamorous one, while Jane’s films have flopped. Unable to establish her talent as an adult actress, Jane has taken to drinking….

You know: rivals.