Short answer. Each co-heir contributes in proportion to his hereditary share, not equally. If a co-heir who should contribute turns out insolvent, the others make up his portion in that same proportion, excluding the share of the heir being indemnified, with a right to reimbursement later if that insolvent heir's finances improve.
What the law says
The reciprocal obligation of warranty referred to in the preceding article shall be proportionate to the respective hereditary shares of the co-heirs, but if any one of them should be insolvent, the other co-heirs shall be liable for his part in the same proportion, deducting the part corresponding to the one who should be indemnified.
Civil Code, Article 1093 — Warranty Is Proportionate. Read the full provision →
What the law says
Those who pay for the insolvent heir shall have a right of action against him for reimbursement, should his financial condition improve.
Civil Code, Article 1093 — Warranty Is Proportionate. Read the full provision →
Contribution follows shares, not headcount
Article 1093 makes clear that the mutual warranty among co-heirs is not split equally person by person. Instead, each co-heir's exposure is measured against the size of the share he actually inherited, so an heir who received a larger portion of the estate answers for a larger slice of the loss, and one with a smaller share answers for less. Two heirs contributing very different amounts toward the same loss is the expected result, not an error.
What happens when a co-heir cannot pay his share
If a co-heir who should contribute toward covering the loss is insolvent, Article 1093 does not let that shortfall fall entirely on the heir being indemnified. The remaining co-heirs step in and cover the insolvent heir's part, again divided among themselves in proportion to their own hereditary shares. The article carves out one exception in that computation: the share belonging to the heir who is actually being indemnified is deducted before working out the others' proportional contributions, so that heir is not made to help pay for his own loss.
The paying co-heirs can later recover from the insolvent one
Covering for an insolvent co-heir is not a permanent write-off. Article 1093 gives the heirs who advanced the insolvent heir's share a right of action against him for reimbursement, but only once his financial condition improves. Until then, the paying heirs simply carry the shortfall themselves; the statute sets no further condition or fixed period for when that improvement has to be shown, only that the claim exists once it happens.
Working out the numbers
In practice this means totaling the hereditary shares of every co-heir bound by the reciprocal warranty, working out what fraction of the estate each one represents, and applying that same fraction to whatever amount is needed to make the affected heir whole. If, for example, one heir holds a fourth of the estate and turns out unable to pay, the remaining heirs absorb that fourth among themselves in proportion to their own respective shares, not split evenly regardless of how much each of them actually inherited.