Short answer. If your father meant the named heirs to take the whole estate but their stated fractions fall short, Article 852 fills the gap by increasing each share proportionally. The leftover is not given to anyone else; it is spread across the instituted heirs in the same ratio as the shares he actually wrote.
What the law says
If it was the intention of the testator that the instituted heirs should become sole heirs to the whole estate, or the whole free portion, as the case may be, and each of them has been instituted to an aliquot part of the inheritance and their aliquot parts together do not cover the whole inheritance, or the whole free portion, each part shall be increased proportionally.
Civil Code, Article 852 — Shares Increased Proportionally. Read the full provision →
What Article 852 provides
Article 852 applies where it was the intention of the testator that the instituted heirs should become sole heirs to the whole estate, or the whole free portion, as the case may be, and each of them has been instituted to an aliquot part of the inheritance and their aliquot parts together do not cover the whole inheritance, or the whole free portion, each part shall be increased proportionally. The trigger is intention. If the testator meant these heirs to have everything, and merely miscalculated the fractions so they fall short, the law honours the intention by enlarging the shares rather than letting a remainder escape to others.
The contrast with a partial will
This is the mirror image of the situation where a testator gives away only part of his estate and the rest passes by intestacy. The dividing line is what he wanted. If he intended to dispose of only a fraction, the untouched part goes to his legal heirs. If he intended the named heirs to be sole heirs of the whole but wrote fractions that do not total one, Article 852 keeps the estate with those heirs and scales their shares up. Same arithmetic — shares under-covering the estate — but opposite outcomes, decided entirely by the intention the will discloses.
How the proportional increase works
The increase is proportional, meaning each heir's share grows in the same ratio it bears to the others, so the balance the testator struck between them is preserved. Suppose he leaves one heir a half and another a quarter — three-quarters in all, a quarter unaccounted for. Rather than splitting the missing quarter equally, the law raises each share by the same proportion, so the two-to-one relationship between the heirs survives and together they now take the whole. The point is faithfulness to the testator's relative preferences: he wanted this heir to have twice that one, and the adjustment keeps it so.
Establishing the intention, and the legitime
Because everything turns on intention, the will is read as a whole for signs that the named heirs were meant to take everything — words like 'sole heirs', or a clear plan to leave nothing outside them. Where that intention is not shown, the partial-will rule and intestacy apply instead, so the reading is not automatic. And as always, the result is checked against the legitime: the shares reserved for compulsory heirs cannot be defeated by a proportional increase among instituted heirs. Settle these in order — establish the intention, apply the increase, then confirm the forced heirs' reserved portions are intact.