Short answer. Only as an advance, and only if the sibling's own separate property is not enough. Support for a brother or sister must first come from that sibling's separate property; the conjugal partnership only advances it if the obligor has none, and later deducts it from that spouse's share on liquidation.
What the law says
In case of legitimate ascendants; descendants, whether legitimate or illegitimate; and brothers and sisters, whether legitimately or illegitimately related, only the separate property of the person obliged to give support shall be answerable
Family Code, Article 197 — Which Property Answers for Support. Read the full provision →
Separate property is the first, and normally the only, source
For a sibling — whether legitimately or illegitimately related to you — the law's starting rule is narrow: only the separate property of the person obliged to give support shall be answerable. That means, as a general matter, support you owe a brother or sister is meant to come out of what belongs to you alone, not out of what belongs to the marriage. This is a stricter rule than the one that applies to a spouse or to your own children.
The conjugal partnership only steps in as an advance
There is a narrow exception, and it is conditional, not automatic. If the person obliged to support the sibling has no separate property, the absolute community or the conjugal partnership "if financially capable, shall advance the support." The key word is advance — the partnership is not permanently shouldering this obligation, it is fronting the money because the obligor has nothing else to draw on.
It gets paid back, eventually, out of the obligor's own share
Whatever the absolute community or conjugal partnership advances for sibling support is not simply absorbed as a shared marital expense. The statute is explicit that the advance "shall be deducted from the share of the spouse obliged upon the liquidation" of the community or partnership. In practical terms, the other spouse's share is protected — the cost of supporting your sibling is ultimately charged against your own eventual share, not split between both of you.
Why this is worth checking before you assume either way
Two conditions both have to be true before conjugal or community funds can even be used: the obligor genuinely has no separate property, and the community or partnership is financially capable of advancing the amount. If the obligor does have separate property, even a modest amount, that property comes first — the marital funds are not a shortcut to avoid touching it. Conversely, if separate property truly is exhausted, the marital estate is not free to refuse; it must advance where it is capable of doing so, subject to being repaid later.