A half sister is a sister who shares one biological parent with you instead of two. If your mom remarried and had a daughter with your stepdad, that daughter would be your half sister because you share the same mom but have different dads. The same goes if your dad had a daughter with someone who isn't your mom: you'd share a dad but have different moms.

The “half” in half sister is just a technical way to describe the biological relationship. It doesn't mean she's only half as important or that you have to feel differently about her. Many people with half siblings never even use the term because the relationship feels exactly like having a sister. After all, you might grow up in the same house, share family traditions, argue over the remote control, and support each other through challenges just like any other siblings.

Some families are straightforward, with kids who all share the same two parents. Other families are more complex, with half siblings, stepsiblings, adopted siblings, and other combinations. A half sister can be your closest confidant, your fiercest competitor at board games, or the person who always has your back when things get tough.