Is it a sin for a young man to abandon a girl he promised to marry in order to please his mother and sisters, and is he obligated to divorce his current wife to please his mother?

There is no sin upon you for not marrying the girl you promised, as long as your breaking of the promise is due to pleasing your mother. However, as for your mother's command to divorce your wife, you are not obligated to obey her in it, nor should you do so if your wife is righteous; because divorcing one's wife is not a form of honoring one's mother.