A stepsister is a girl who becomes your sister through marriage, not by sharing parents. This happens when your parent marries someone who already has a daughter.

For example, if your mom marries someone who has a daughter from a previous relationship, that daughter becomes your stepsister. You don't share biological parents, but your families have joined together. Similarly, if your dad marries someone with a daughter, that girl is your stepsister.

The relationship can feel different for different people. Some stepsisters become as close as biological siblings, sharing rooms, secrets, and inside jokes. Others remain more like friendly housemates. There's no single right way for the relationship to develop, and it often takes time to figure out how you fit into each other's lives.

Other similar terms include stepbrother, stepparent, and stepchild.