I really think it depends on the situation. It is hard for younger children to make that distinction especially when other half-siblings call the step-parent "Mommy" or "Daddy". In my situation my husband and BM were never married. Since my husband and BM did not have a committed relationship, I have been with my husband since before my SD was born. Being there her entire life and then having her half-brother (our son) born when she was 3 really makes calling me "Mommy" harder to avoid. But now that she is 9, she does understand the distinction between Mommy me and Mommy BM, but still continues to call me Mom as well because that is what she is comfortable with. I think in the end that is what is the most important - the child's comfort level. However, it can be hard when they're younger because they just don't fully understand. Also when there are multiple boyfriends or girlfriends it can get confusing and frustrating for parents as well as the children caught in the middle.