Short answer. Generally no. Under Article 1403, a special promise to answer for the debt, default or miscarriage of another is covered by the Statute of Frauds and is unenforceable by action unless it is in writing and signed by the party charged. A purely verbal guarantee normally cannot be sued upon.
What the law says
A special promise to answer for the debt, default, or miscarriage of another
Civil Code, Article 1403 — Unenforceable Contracts and the Statute of Frauds. Read the full provision →
A promise to answer for another's debt needs a writing
Article 1403 lists the agreements that the Statute of Frauds requires to be in writing. Among them is A special promise to answer for the debt, default, or miscarriage of another. For these, the article says the agreement is unenforceable by action unless the same, or some note or memorandum, thereof, be in writing, and subscribed by the party charged, or by his agent. So your spoken promise to cover your brother's debt is exactly the kind of undertaking the law wants documented. Without a written, signed note, a court will not compel you to make good on a purely verbal guarantee of someone else's obligation.
Why guarantees are singled out
The law is cautious about promises to pay for another person's failings because they are easy to assert falsely and serious in consequence. A guarantor takes on liability for a debt that is not his own, often as a favor, and memories of casual conversations can be unreliable or self-serving. Requiring a writing signed by the person to be charged guards against someone being trapped into another's debt on the strength of a disputed verbal exchange. The party charged — here, you — must have put the commitment, or a memorandum of it, in signed writing before it can be enforced against you by suit.
What the rule does not mean
Being unenforceable is not the same as being void or illegal. A verbal guarantee that fails the Statute of Frauds simply cannot be enforced by court action while it stays unwritten and unratified; it is not a crime to have made it. The article itself says these contracts are unenforceable unless they are ratified. Ratification — for instance, by accepting benefits under the promise or otherwise confirming it — can cure the defect. And the requirement does not touch your brother's own liability for his debt; it concerns only whether your separate promise to answer for it can be sued upon.
How this applies to you
If all you gave was a spoken assurance to answer for your brother's debt, the creditor generally cannot force you to pay on that basis alone. Your protection lasts only so long as there is no signed writing and no ratification, so be mindful that later conduct confirming the promise could change the picture. This rule also assumes your promise was truly to answer for another's debt rather than to assume the obligation as your own primary undertaking, which can be treated differently. On the facts as you describe them, a bare verbal guarantee is not enforceable by action.