Stepfamilies contain relationships at different stages of development at the same time. Most friction traces to a single question: who is entitled to direct whom.
The relationships are unevenly established
The adult couple has chosen each other and has a developed relationship. The children did not choose the arrangement and have relationships of very different depth with each adult.
Expecting all these relationships to move at the same pace produces predictable strain, because a stepparent and a child are effectively strangers who share a house.
Time is the main variable, and the pace is set by the child rather than by the adults' schedule for the family to feel settled.
Authority and affection are separate questions
A common failure is a stepparent assuming a disciplinary role early, which invites the response that the adult has no standing to enforce it.
Arrangements that work more often keep discipline with the biological parent initially, with the stepparent holding a supportive role closer to a trusted adult relative.
Authority then accrues gradually as the relationship develops, rather than being asserted on the basis of the marriage.
Two households mean two rule sets
Children moving between households encounter different bedtimes, screen rules, chores and expectations, and consistency across households is rarely achievable.
Children generally manage this better than adults expect, in the same way they manage different rules at school and at home.
What causes damage is not the difference but conflict about it, particularly when a child is asked to report on or evaluate the other household.
Loyalty pressure is the underlying stress
Children often feel that warmth toward a stepparent is disloyalty to a parent, and they resolve that tension by keeping some distance.
Adults who read the distance as rejection sometimes respond by pressing harder, which increases the loyalty conflict rather than resolving it.
Explicitly relieving the pressure, by making clear that no one is being replaced, addresses the actual difficulty more effectively than more shared activities.
Structure reduces the number of live disputes
Written agreements about schedules, holidays, expenses and communication remove recurring negotiations that otherwise reopen every few weeks.
Regular check-ins between the adult partners, held privately rather than in front of children, keep small disagreements from being resolved in the moment and in public.
Family therapists with stepfamily experience work on these patterns specifically, and custody and support arrangements vary by state, so a family law attorney is the right source on legal questions.