Must I guarantee the debt I lent from the inheritance money to a person who passed away, and whose heirs refused to repay the debt, and must I return it to my brothers and sisters?

The agent is a trustee, and his trustworthiness necessitates his complete adherence to what is agreed upon with the principal. If the remaining heirs authorize the agent to lend the money, then the agent is not a guarantor of it, especially if he takes measures to document the debt, such as writing and witnesses. However, if they do not authorize him, then the agent is a guarantor of the money and must return it to the inheritance or have it deducted from his share of the inheritance.