You asked

Why do siblings fight and what can I do to stop them?

If your primary school child is the baby of the family, by now he will have figured out that his older siblings get more independence and privileges than him. Your older child will quickly discover that he doesn’t want his younger sibling tagging along wherever he goes. This could result in name-calling, tattling, pushing and hitting by the older sibling. This could keep your children busy for hours while you become more and more in danger of losing your sanity!
The fact is some siblings will spend their whole lives embroiled in disagreements and spats so you should accept that the occasional shouting match is unavoidable. However, it’s a good idea to teach your child the importance of treating each other respectfully and in resolving their own conflicts.

More questions

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Many parents will be all too familiar with siblings getting embroiled in all sorts of scrapes and squabbles. 
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If you have a problem with your child being aggressive, follow some of these tried and tested tips: