The case you've described seems complex, however, there are a few legal principles that may be applicable, specifically in the context of Saudi Arabian law, which is largely based on Islamic jurisprudence (Sharia Law).
1. The Law of Procedure before Sharia Courts:
According to Article 13 of The Law of Procedure before Sharia Courts, "No one shall be held liable for the liabilities of another unless he undertakes such liabilities either expressly or impliedly." So, from a general standpoint, every person is responsible for their own liabilities.
2. Commercial Pledge Law:
If you signed a guarantor agreement for your brother's loans, under the Saudi Commercial Pledge Law, Royal Decree No. M/49 dated 13/8/1433H (corresponding to 2 July 2012), you are generally held responsible for his debt to the bank.
3. Saudi Bankruptcy Law:
As per "Saudi Bankruptcy Law" Royal Decree No. M/05 dated 28/5/1439, there are certain procedures such as financial restructuring, settlement, or liquidation, which may be applicable based on the state of the company and its ability to repay the debt.
Nevertheless, your situation is complex and potential defenses or arguments could be existence of fraud, undue influence, error in consent when the guarantor agreement was signed or breach of agreement by the bank or your brother, if any.
You would need to consider these aspects and each of these laws in detail to form a strong defense. Furthermore, considering the international nature of your situation being a U.S. citizen now, international laws and treaties might also apply.
However, it is essential to consult with a lawyer who is specifically familiar with Saudi Arabian law to examine all the relevant facts and documents in your case. They would be able to provide a much more accurate reading of your situation and give you the best possible advice. Keep in mind that this is a general guidance and not legal advice.