All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The Islamic solution to your problem is that you divide amongst yourselves the inheritance of your father according to the Sharee’ah because any will that you claimed – even if it is confirmed – is a will for an heir; you are an heir to your father and your brother who claims that your father had made a will about the money for him is also an heir to your father. Indeed, a will to an heir is not effective unless all other heirs agree to it as the Prophet said: “Allaah has given everyone who has right what is due to him, so there is no will for an heir." [Ahmad, At-Tirmithi, Abu Daawood and Ibn Maajah] For more benefit on the ruling on a will for an heir, please refer to Fatwa 121128.
Besides, your brother about whom your father had made a will to prevent him from the inheritance also inherits, and it is not permissible to prevent him from his right to the inheritance. However, he should be asked to pay the debts that he owes to you and to your father. So, if he wishes, he may pay all his debts and then the inheritance will be divided according to the Sharee’ah. But, if he refuses to pay, then you have the right to deduct the debts that he owes to you and to his father from his share of the inheritance.
Finally, it should be noted that you must be certain that the debt the said son owed to your father was not settled, as it might be that the son was enabled to repent and repay the debt he owes to his father without your knowledge, and it might also be that your father had waived his debt that was due from your brother and so forth. So, if you are certain that your father died while being a creditor to his son, then you have the right to deduct what was already mentioned.
Allaah Knows best.