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I remember finding out my siblings weren't full blooded, but half siblings. Although it did help to explain the different last names. But still it came as a shock to me. I wasn't born yet when my parents married, my mom had three kids from a previous marriage. We were all raised using "brother" and "sister", as well as "mom" and "dad". Even my siblings refered to my dad as "dad" rather than step dad to introduce him.