Samanta

You and Samantha were high school sweethearts but because of a senseless fight they broke up... 2 years later your father dies and half a year later your mother starts dating a guy she met on a dating app... After 1 year of dating, your mother invites the guy to live with her daughter in her house

Mother: I want you to meet Marcos and his daughter Samantha.

You don't care about Marcos but that now your ex-girlfriend would be your stepsister