Short answer. You can raise the challenge, but it depends on whether the false reason was decisive. Article 850 treats a false cause for an institution as simply not written — leaving the institution intact — unless the will itself shows your father would not have chosen that heir had he known the cause was false.
What the law says
The statement of a false cause for the institution of an heir shall be considered as not written, unless it appears from the will that the testator would not have made such institution if he had known the falsity of such cause.
Civil Code, Article 850 — A False Cause for the Institution. Read the full provision →
The default rule — false cause does not void the institution
Article 850 of the Civil Code establishes a careful default: when a testator states a false reason for instituting an heir, the statement is treated as not written, but the institution itself stands. The logic is that a testator's choice of heir is a personal and deliberate act. The reason given in the will is not always the real reason, and striking down an institution every time a stated reason was inaccurate would undermine too many legitimate testamentary choices. Your brother's institution survives unless you can do more than prove the stated reason was wrong.
When the false cause becomes decisive
The rule has an exception: the statement of a false cause for the institution of an heir shall be considered as not written, unless it appears from the will that the testator would not have made such institution if he had known the falsity of such cause. The critical phrase is unless it appears from the will itself that the testator would have acted differently. You cannot just bring in witnesses or other extrinsic evidence to argue what your father really meant. The will must demonstrate, on its face, that the institution was conditioned on the stated reason being true.
What to look for in the will's language
The distinction turns on the words your father used. A will that says "I institute my son because he has helped manage the family business" treats the stated reason as background — the institution stands even if the background was wrong. A will that says "I institute my son only because he has helped manage the family business, and if that were not true I would institute my daughter instead" makes the cause decisive and conditional. Review the exact language in the will carefully. The closer the text comes to tying the institution directly and exclusively to the false condition, the stronger your ground for challenge.
Practical implications for your situation
Challenging an institution under Article 850 is not straightforward. You need to demonstrate both that the stated cause was false and that the will itself — not outside accounts of your father's intentions — reveals that he would not have made the same choice without it. If the will's language is neutral or ambiguous, the default favors leaving the institution in place. Gathering the exact text, understanding what the will says about your father's reasons, and evaluating how clearly those reasons were tied to the institution are the starting points for assessing whether Article 850's exception applies.