Families are strengthened by having goals. A new step parent might seek to discover the goals and dreams of each child. They can support and encourage each child's dreams. Over time the parents can invite all family members to think about common goals. Family projects can be a catalyst for unity.
Family fun can build closeness.
One of the dangers for step parents is taking sides. It is natural to develop stereotypes about our children and step children. Stereotypes prevent us from helping. Thinking win-win is more likely to lead to helpful relationships. When children are in conflict, a parent or both parents can meet with each combatant individually.