Short answer. Your father's legal heirs. Article 1022 says that in testamentary succession, when accretion does not take place and no substitute has been designated, the vacant portion of the instituted heirs passes to the testator's legal heirs, who receive it with the same charges and obligations attached.
What the law says
In testamentary succession, when the right of accretion does not take place, the vacant portion of the instituted heirs, if no substitute has been designated, shall pass to the legal heirs of the testator, who shall receive it with the same charges and obligations.
Civil Code, Article 1022 — When Accretion Does Not Apply. Read the full provision →
This is the fallback when both other mechanisms fail
Testamentary succession normally has two ways of dealing with a share that an instituted heir cannot or will not take: a substitute the testator named to step in, or accretion, which lets other heirs of the same disposition absorb the vacant portion automatically. Article 1022 addresses what happens when neither of those applies — when the right of accretion does not take place, and if no substitute has been designated. With both of the usual mechanisms unavailable, the share does not simply remain in limbo.
The vacant share passes to the testator's legal heirs
In that situation, Article 1022 directs that the vacant portion shall pass to the legal heirs of the testator — meaning the heirs who would inherit under the rules of intestate succession, as though the testator had left no will covering that particular portion. This treats the unfilled share as effectively falling outside the will's testamentary scheme and reverting to the default intestate distribution, rather than leaving it unclaimed or forcing it onto heirs who were never meant to receive it.
The legal heirs take the burdens along with the benefit
The article does not let the legal heirs take this vacant portion free and clear of whatever conditions attached to it. They receive it with the same charges and obligations that would have applied to the instituted heir who was originally meant to take that share. Any conditions, legacies to be paid out of it, or other burdens the testator attached to that specific portion travel with it to the legal heirs now receiving it, rather than being extinguished simply because the original beneficiary never took the gift.
A benefit that comes with strings
For the legal heirs, the practical effect is a windfall tempered by strings: they receive a portion they were never named to take in the will, but they take it exactly as burdened as the instituted heir who was originally meant to have it. The other instituted heirs gain nothing from the lapse, and any conditions or legacies the testator attached to that share continue to bind whoever now takes it, so the portion arrives useful but not unencumbered.