Siblings are not arguing about fairness
Sibling conflict presents as accounting. He got more. She went first. That is not fair. Adults, taking the complaint at face value, respond with measurement — cutting things in half, alternating turns, keeping score.
Measurement escalates. Once fairness is established as the currency, every division becomes a place to audit, and the children get better at auditing than the adult can be at dividing.
Answering a fairness complaint with a more precise division teaches that precision is what was being asked for.
What is usually underneath
Most of these disputes are about access to the adult, not about the object. A child who has had ten minutes of attention typically stops counting; a child who has not will find something to count.
Practical
Answer the need rather than the claim where you can. "You want a turn with me" ends more arguments than any equal division of biscuits, and it is often true even when the child would deny it.