Short answer. No. A will cannot hand the sizing of the shares to somebody else. When the heirs are named in the will, the testator himself must fix how much each one takes; a clause leaving that decision to your eldest brother, or to any third person, is void as to that delegation.
What the law says
the determination of the portions which they are to take, when referred to by name, cannot be left to the discretion of a third person
Civil Code, Article 785 — Designation Cannot Be Delegated. Read the full provision →
Why the law refuses to let someone else decide
Making a will is treated as a strictly personal act. The whole point of the formalities — the writing, the witnesses, the signature — is that the document expresses what the testator wanted, tested at the moment he was still able to say so. If the sizing of the shares could be handed off to a living person, the real disposition would be made after death by someone the law never examined for capacity or free will, and never required to sign anything. So the Civil Code closes the door: the duration or efficacy of the designation, and the determination of the portions, cannot be left to a third person's discretion when the heirs are referred to by name.
What this does not stop your father from doing
The rule bites on discretion, not on arithmetic. Your father may still divide unequally, so long as he does the dividing — half to one child, a quarter each to two others, a specific lot to a fourth. He may attach conditions that operate on their own terms, and he may name an executor to gather, value and hand over the estate. An executor who merely carries out a division the will already fixed is not deciding anything. The line is crossed only when the will effectively says let X judge who deserves more. A designation tied to a fact that can be verified from outside — an existing debt, a share already advanced — is measurement, not delegation.
What happens to the will if such a clause is written anyway
A defective delegation does not automatically destroy the whole will. The usual consequence is that the offending clause fails and the rest of the document stands, with the estate distributed under the rules that apply when a portion is left undisposed. That normally means the compulsory heirs still take their legitimes, and whatever your father did not validly allocate is divided according to the intestate rules among those entitled. In practice the result is often close to equal shares among the children — which is frequently the opposite of what the person who inserted the clause was hoping for.
If you are facing this now
A will only takes legal effect once it has been probated, and objections of this kind are raised in that proceeding rather than in a family meeting. Do not sign a deed of extrajudicial settlement that simply adopts your brother's allocation on the strength of a clause like this; a signature can be read as your own agreement to the split, quite apart from whether the clause was valid. Keep the original will and any drafts, note who was present when it was prepared, and get the document reviewed before any distribution is executed. Every estate turns on its own facts, and nothing here predicts how a particular one will come out.