Short answer. The surviving spouse's legitime is one-fourth of the hereditary estate. This applies specifically because your sibling left no legitimate descendants but did leave legitimate ascendants — your parents — and that fourth is taken from the free portion of the estate rather than from the ascendants' own legitime.
What the law says
If the testator leaves no legitimate descendants, but leaves legitimate ascendants, the surviving spouse shall have a right to one-fourth of the hereditary estate. This fourth shall be taken from the free portion of the estate.
Civil Code, Article 893 — Spouse With Legitimate Ascendants. Read the full provision →
The rule that applies to your sibling's situation
This article addresses exactly this combination of survivors: no legitimate descendants, but legitimate ascendants who survive. Your parents, as your sibling's legitimate ascendants, put this situation squarely within this provision. Where that combination exists, the surviving spouse's legitime is fixed at one-fourth of the hereditary estate — a specific fraction set out for this exact configuration of heirs, distinct from the rules that apply when descendants are the ones who survive alongside the spouse.
Where that fourth comes from
The statute specifies that the spouse's one-fourth share is taken from the free portion of the estate — the part the deceased could have freely disposed of by will — rather than from the ascendants' own legitime. This matters because it means the spouse's share does not reduce what the ascendants are separately guaranteed under the rules governing their own legitime; the two shares are drawn from different parts of the estate.
What determines whether this article applies
The key condition is the absence of legitimate descendants combined with the presence of legitimate ascendants. If your sibling had also left a legitimate child, this specific one-fourth rule would not apply, since a different provision governs the spouse's legitime when descendants survive. Likewise, if there were no surviving ascendants either, the spouse's legitime would be determined under whatever rule governs a spouse who is the sole compulsory heir, rather than this one. Whether the estate's other assets are enough to actually satisfy both the ascendants' legitime and this fourth depends on the total value of the estate, which this provision does not itself calculate. It also does not address illegitimate children or siblings of the deceased, whose presence, if any, is governed by other rules on legal succession entirely.
How this differs from the descendants scenario
When legitimate descendants survive instead, the spouse's legitime is measured differently — as either a flat one-fourth where a single child survives, or as a share equal to each child's legitime where two or more descendants survive. The rule for ascendants set out here is simpler by comparison: it fixes one-fourth regardless of how many legitimate ascendants actually survive, whether that is one surviving parent or both. The number of ascendants does not change the spouse's fraction the way the number of descendants can in the descendants-only scenario.