Short answer. Possibly. The Civil Code generally protects a testator's own partition from being challenged for mere inequality, unless the legitime of a compulsory heir is prejudiced, or it appears — or may reasonably be presumed — that the testator actually intended something else.
What the law says
The partition made by the testator cannot be impugned on the ground of lesion, except when the legitime of the compulsory heirs is thereby prejudiced, or when it appears or may reasonably be presumed, that the intention of the testator was otherwise.
Civil Code, Article 1099 — Partition by the Testator. Read the full provision →
The general protection this article gives a testator's partition
Article 1099 starts from a position of deference to the testator: a partition the testator made cannot be attacked on the ground of lesion — meaning that a share turned out to be worth less than it should have been. An uneven division, by itself, is not a defect the law lets an heir undo. The article then carves out two specific exceptions where a challenge is allowed.
The first exception: the legitime is prejudiced
The first ground for impugning the partition is that the legitime of the compulsory heirs is prejudiced by it — meaning the uneven division actually cuts into the portion the law reserves for compulsory heirs, not merely that shares differ. If your uneven share genuinely falls below your legitime, that is a distinct, independent ground for challenging the partition, separate from any question about the testator's intent.
The second exception: a reasonably presumed contrary intention
The second ground is the one your situation raises directly: the partition may also be challenged when it appears, or may reasonably be presumed, that the testator's actual intention was different from what the partition as written provides. This does not require you to prove the testator's true intent with certainty — the word "presumed" signals that reasonable inference from the will and surrounding circumstances can be enough, but it still has to be a reasonable presumption, not mere speculation that equal shares would have been fairer.
What this means for your situation
Whether an uneven division "looks like" a mistake is not, by itself, the legal test Article 1099 sets — the question is whether the will and surrounding circumstances reasonably support presuming the testator meant something other than what was written, or whether the unevenness actually prejudices someone's legitime. Where the facts genuinely support either of those two grounds, Article 1099 allows the partition to be challenged; where they do not, the general rule protecting the testator's own division controls, however unequal the final shares may look on paper.