Observational studies of young siblings have recorded conflict at rates that surprise people — frequent enough that intervening in each instance would occupy most of a day.
Which is worth knowing before deciding how to respond to it.
Why it happens
Siblings compete for finite resources: parental attention, possessions, space, and status within the family.
They also spend enormous amounts of time together, in a relationship they didn't choose and can't leave, with no social conventions restraining them. Children are polite to friends because friends can walk away. Siblings can't.
That combination — high contact, real competition, no exit — produces conflict reliably.
What it may be doing
Sibling relationships are one of the few contexts where children practise conflict with an equal who won't simply accommodate them.
Research on sibling relationships has associated them with development of social understanding, perspective-taking and conflict resolution skills.
The mechanism proposed is that negotiating with a sibling requires understanding another person's position, arguing a case, and reaching some resolution — with someone who has their own agenda and won't defer.
Adults intervening in every dispute removes that practice.
What intervention does
Several unhelpful effects.
It provides attention. Conflict that reliably produces parental involvement becomes a route to attention, particularly if attention is otherwise scarce.
It establishes a court of appeal. If a parent decides disputes, children stop trying to resolve them and start litigating instead. The skill being practised becomes persuading a parent rather than negotiating with a sibling.
It creates roles. Repeatedly framing one child as the aggressor and one as the victim tends to entrench those positions.
It's frequently based on inadequate information. Parents typically arrive after the start and see the response rather than the provocation.
When to intervene
Not never, and the threshold should be higher than most people set it.
Physical danger. Immediately and without discussion.
Genuine imbalance. A significant age or size difference means it isn't a negotiation.
Sustained cruelty rather than conflict. Persistent belittling, exclusion or humiliation is different from arguing over a toy, and it warrants attention.
When they ask. If a child seeks help, that's different from a parent inserting themselves.
How to intervene when you do
Approaches that work better than adjudicating.
Describe rather than judge. Stating what you see — two people who both want the same thing — without assigning blame.
Hand the problem back. Acknowledging it's difficult and expressing confidence they can work something out. Then leaving.
Remove the object. If a dispute is over an item and shows no sign of resolving, the item can be unavailable for a while. Simpler than determining who was right.
Separate without punishing. Time apart to calm down, framed as a break rather than a sanction.
Avoid investigating. Determining who started it produces a competition to construct a better account, which is not the skill you're hoping to develop.
The fairness trap
Children monitor fairness with considerable attention, and attempting to achieve exact equality is both impossible and counterproductive.
Identical treatment isn't the same as fair treatment. Children of different ages have different needs and different capacities.
The approach that tends to work better is meeting individual needs and being able to explain the reasoning, rather than aiming for identical outcomes. A child who understands why their sibling gets a later bedtime accepts it more readily than one told they're being treated equally when they visibly aren't.
Comparison
Worth naming specifically because it's a strong predictor of poor sibling relationships.
Comparing children, even favourably, sets them against each other. Praise framed relative to a sibling is heard as a ranking.
Research on parental differential treatment has found associations between perceived favouritism and both sibling conflict and individual wellbeing.
Complete equality of treatment isn't achievable and isn't the point. Avoiding explicit comparison is achievable and worth the effort.
The long view
Sibling relationships change enormously over time. Childhood conflict is a poor predictor of the adult relationship, and many people who fought constantly as children are close as adults.
What does appear to matter is whether the relationship contains warmth alongside the conflict, and whether parental behaviour entrenched positions or allowed them to shift.
Which suggests the useful goal isn't reducing conflict but ensuring there's something else there too — shared activities, occasions when they're on the same side, and space for the relationship to be theirs rather than something managed by adults.
Age gaps, briefly
A question parents ask constantly and the research answers unhelpfully: no particular gap is reliably better.
Closer gaps tend to produce more conflict in early childhood alongside more shared play and shared interests. Wider gaps tend to produce less direct competition and less common ground.
Studies looking for relationships between age gap and long-term sibling relationship quality have not found consistent effects large enough to guide a decision.
Which is worth knowing mainly as reassurance for anybody worrying that they got the timing wrong. The evidence suggests the timing was not the determining factor.