Short answer. In proportion to the resources of each, not in equal shares. Where the obligation to give support falls on two or more people, the Family Code divides the payment according to what each can afford, so the better-off obligor carries the larger part of it.
What the law says
When the obligation to give support falls upon two or more persons, the payment of the same shall be divided between them in proportion to the resources of each. However, in case of urgent need and by special circumstances, the judge may order only one of them to furnish the support provisionally, without prejudice to his right to claim from the other obligors the share due from them.
Family Code, Article 200 — Dividing the Support Obligation; Competing Claims. Read the full provision →
Proportion to resources, not equal shares
The default is not a headcount. Payment is divided between them in proportion to the resources of each, which means two obligors of very unequal means do not pay the same amount. Resources here is a broader idea than monthly salary; it takes in what a person actually has available after their own legitimate obligations. A sibling supporting three children of their own is not in the same position as one with none, even on identical pay. The division is a judgment about capacity, and it can be revisited when capacity changes.
The urgent-need shortcut
Apportionment takes time, and a person needing support usually cannot wait for it. So in case of urgent need and by special circumstances, the judge may order only one of them to furnish the support provisionally. That is a device for speed, not a finding that the chosen obligor owes the whole. It is expressly without prejudice to his right to claim from the other obligors the share due from them, so the one who advanced the money can recover the others' shares afterwards. An obligor asked to shoulder everything provisionally should keep clean records for exactly that reason.
The mirror problem: one payer, several claimants
The article also covers the reverse situation, where two or more recipients claim support at the same time from one person who cannot satisfy them all. The general solution is to follow the order of preference the Code establishes among relatives. There is one express override: where the competing claimants are the spouse and a child subject to parental authority, the child shall be preferred. That rule decides a very common real-world conflict, and it decides it in the child's favour regardless of who filed first or who is in greater apparent distress.
What a claim of this kind needs
Because the division turns on relative means, the case is largely built out of the obligors' finances rather than your own hardship. Identify every person on whom the obligation falls, not just the most approachable one, since leaving an obligor out shifts their share onto the others and invites a dispute later. Then assemble what can be shown of each one's income, property and existing burdens, together with a clear statement of what the support is actually needed for. Take that to a lawyer before choosing whom to proceed against.