The novel focuses on a girl, she is the daughter of a concubine in her first life she married the boy her sister did not want and ended up being loved and happy all her life. Her sister goes back in time and regrets it, so the sister marries her husband from the first life and she marries another man in this second life.
Might be Transmigrating into the Female Supporting Character with a Good Life in a Laid-Back Novel. Although, if I recall, in this one the regretful older rebirthed (and transmigrated) sister doesn't end up marrying the original husband despite trying to.