Short answer. Yes, generally. Article 2164 gives a stranger who furnishes support, without the knowledge of the person legally obliged to give it, the right to claim reimbursement from that obligated relative, unless it appears you gave the support out of piety and without any intention of being repaid.
What the law says
When, without the knowledge of the person obliged to give support, it is given by a stranger, the latter shall have a right to claim the same from the former, unless it appears that he gave it out of piety and without intention of being repaid.
Civil Code, Article 2164 — Support Furnished by a Stranger. Read the full provision →
You can generally claim it back from the obligated relative
Article 2164 addresses exactly this kind of situation, where someone steps in to support a person while the relative who was actually bound to do so does nothing, unaware that you had taken over. It provides that when, without the knowledge of the person obliged to give support, it is given by a stranger, the latter shall have a right to claim the same from the former. Because you provided that support without the relative's knowledge, you generally have a right to be reimbursed by the relative who was legally bound to provide it in the first place.
The exception for support given out of piety
This right to reimbursement is not absolute. The same article carves out a real exception: it does not apply if it appears that he gave it out of piety and without intention of being repaid. If the circumstances show you provided the support purely out of compassion or a sense of duty, with no expectation of ever getting the money back, the law respects that as a genuine act of generosity rather than treating it as a claim you can later change your mind about.
Why the law distinguishes intent this way
Support is meant to be someone's legal responsibility, and when a stranger fills that gap, the underlying obligation has not disappeared; it has simply been advanced by someone else. Ordinarily, that stranger should be able to recover what they spent from the person who was truly obligated. But forcing reimbursement on someone who clearly intended their help as a gift, given out of compassion rather than expectation of repayment, would distort what was actually a voluntary, charitable act into an involuntary loan.
What this means for your reimbursement claim
If you supported the person without the obligated relative's knowledge, and you did not intend it as a gift given purely out of piety, you have a basis under Article 2164 to seek reimbursement from that relative for the support you furnished. Being able to show your intent at the time, that you expected to be repaid rather than simply helping out of compassion, strengthens your position if the relative disputes the claim.