Short answer. No. The law is explicit: the cause of unworthiness shall be without effect if the testator had knowledge thereof at the time he made the will. If your father knew of the wrongdoing before writing the will and still named your brother, that specific ground cannot later be used against him.
What the law says
The cause of unworthiness shall be without effect if the testator had knowledge thereof at the time he made the will
Civil Code, Article 1033 — Condoning Unworthiness. Read the full provision →
Prior knowledge neutralizes the ground at the moment the will is made
The provision ties the effect of a cause of unworthiness directly to what the testator knew, and when. If the testator had knowledge thereof at the time he made the will, that cause is deemed without effect. The knowledge does not need to be expressed in any particular form or accompanied by a written statement to have this consequence — the statute conditions the outcome purely on knowledge existing at the exact moment of making the will, nothing more and nothing less than that timing.
Why naming the heir despite knowing is treated as decisive
The logic behind this rule is that a testator who already knew of the wrongdoing and still chose, in the will itself, to name that person as heir has effectively made a considered decision that stands. The law does not let someone else later reach back and use a ground the testator himself was already fully aware of, and evidently did not treat as disqualifying, at the very moment he actually sat down and wrote the document naming his heirs.
This is different from the second scenario in the same article
The same provision separately addresses a testator who learns of the cause subsequently — after the will was already made — and specifies that in that situation, the cause is likewise without effect only if the testator condone[s] them in writing. Your facts fit the first scenario described, not the second one: your father's knowledge of the wrongdoing clearly predates the will, so the separate written-condonation requirement that applies specifically to after-the-fact discovery simply does not come into play in your case at all.
What this means for your brother's status as heir
Because your father's knowledge of the wrongdoing existed before he made the will, and he still named your brother as heir in it anyway, this particular ground for declaring your brother unworthy to inherit is one the law now treats as already neutralized. Whether some other, entirely separate ground for unworthiness might exist independently is a distinct question this specific fact pattern does not resolve one way or the other.