How do siblings split the money fairly?

Decide it before the first dollar arrives, because splitting money after it exists is a negotiation and splitting it beforehand is just a rule.

Equal is the obvious answer and it is only fair when the contribution is genuinely equal, which it rarely is. One of them found the customer. One of them owns the equipment. One does the part nobody wants. An even split that ignores all of that produces exactly the resentment it was meant to prevent.

Three structures that work better.

By the work. Whoever does the job gets the money for that job. Simplest, cleanest, and it handles the very common case where one sibling is enthusiastic and the other is occasional.

One hires the other. It is one kid's business and the other is paid per job or per hour. This is honest, it removes most of the arguments, and everybody knows whose thing it is. Around a third of the job is a reasonable rate for a helper.

By role, with a split agreed up front. If the roles are genuinely different and both are doing real work, an uneven split can be fair. Sixty forty where one found the customers and does the scheduling is not unjust, it is accurate.

What matters more than the formula is that it was agreed in advance and written down. Four lines is enough. Kids will accept a split they think is slightly unfair if they agreed to it, and will resent an objectively fair one that got decided after the fact.

Costs come out first, before anything is divided. Fuel and supplies are not profit, and a kid who takes their half off the top before expenses is going to be short when something needs replacing.

Then let them argue about it a bit. Working out a fair split with somebody you cannot walk away from is a useful thing to practice at this age.