Couple relationship predates parent-child relationship. Family roles and rules evolve. Biological parents have legal rights regarding children. Both biological parents live in household. Children live in one household.
Children arrive one at a time, over time.
The Stepfamily Family born of loss with death or divorce of parent. Two fully developed, and often diverse, sets of traditions. Biological parent-child bond predates remarried couple relationship. Family roles and rules confused and unclear. Stepparents of stepchildren lack legal relationship.