Sibling relationships receive far less research attention than parent-child ones and are frequently more consequential, being longer and involving more hours of interaction in childhood.
Conflict frequency
Observational studies of young siblings find conflict at rates that surprise parents when quantified — multiple disputes per hour in some studies.
Which is normal, and its normality is worth knowing since parents frequently interpret it as a failure.
Conflict declines with age and changes character, becoming less physical and more verbal.
What conflict may do
Sibling conflict provides practice in negotiation, perspective-taking and emotional regulation in a relationship that cannot be abandoned.
Which is a proposed developmental function, with correlational support and limited causal evidence.
The distinction that appears to matter is between conflict that is resolved and conflict that is destructive, with the second associated with worse outcomes.
Parental intervention
Research suggests that how parents intervene matters more than whether they do.
Mediation — helping children articulate positions and find solutions — is associated with better outcomes than either imposing a resolution or ignoring the dispute.
Which takes longer in the moment and appears to reduce subsequent conflict.
Consistently taking one child's side, particularly the younger one, is associated with resentment.
Differential treatment
Children are acutely sensitive to differences in parental treatment.
Perceived favouritism is associated with poorer outcomes for both the disfavoured and, in some studies, the favoured child.
Which is complicated by the fact that different children genuinely need different things, and treating them identically is neither possible nor appropriate.
The finding that appears to matter is whether children perceive the differences as fair, which is affected by whether they are explained.
Birth order
Popular claims about personality by birth order have been examined in very large datasets.
The consistent finding is that effects on personality are essentially absent or extremely small.
A small effect on measured intelligence favouring earlier-born children has been found in some large studies and is very small in magnitude.
Which means the elaborate birth order personality frameworks are not supported.
Age gaps
Research finds no single optimal gap.
Closer gaps are associated with more conflict and more shared interests. Wider gaps with less of both.
Which means the trade-offs differ rather than one being better, and practical family considerations reasonably dominate the decision.
Siblings of children with additional needs
Research finds varied outcomes, including both elevated difficulties and, in several studies, greater empathy and maturity.
Which appears to depend substantially on family support, information and whether the sibling's own needs receive attention.
Support services specifically for siblings exist in many areas and are underused.
Adult sibling relationships
Frequently improve after childhood, and childhood conflict does not predict adult relationship quality strongly.
Which is reassuring for parents watching daily disputes, and it is a genuine finding rather than a comforting assumption.
Persistent aggression, bullying between siblings or any concern about a child's safety warrants professional advice, since sibling aggression at the severe end has been associated with the same outcomes as peer bullying.
New babies
The arrival of a sibling is a significant adjustment, and regression in behaviour is common and temporary.
Which is normal and generally resolves, and preparation before the birth is associated with easier adjustment.
Involving an older child in care, at a level they can manage, is generally recommended.
Sharing
Genuine sharing requires perspective-taking that develops gradually.
Which means insisting on sharing from a very young child requires something they cannot yet do.
Turn-taking with a defined end, rather than open-ended sharing, works better developmentally.
Individual attention
Time with each child separately is consistently identified by families as valuable and is difficult to arrange.
Which need not be substantial — regular short periods appear to matter more than occasional long ones.
Comparison
Comparing siblings, even favourably, is associated with worse relationships between them.
Which is easy to do inadvertently and worth noticing.
Twins and multiples
Have a distinctive relationship with its own research literature.
Questions about separate or shared classes, individual identity and comparison arise specifically.
Which are addressed differently by different families, and evidence on classroom separation is mixed rather than favouring one approach.
Only children
Research has consistently failed to find the deficits that stereotypes assert.
Which has been examined extensively, with meta-analyses finding no meaningful differences in adjustment, sociability or character.
A small advantage in achievement and in parent-child relationship quality appears in some analyses.
Blended families
Step-sibling relationships take time to develop and forcing closeness generally slows it.
Which is a consistent finding in the research, and expectations of immediate family feeling are the common source of difficulty.
Long-term support
Siblings frequently become primary sources of support in adulthood, particularly during parental illness and bereavement.
Which is worth knowing during years when the relationship looks purely adversarial.
Which is a reasonable long view to hold on days when the immediate evidence points the other way.