You need to explain to your father, in the letter, that if he leaves everything to your daughter in the manner that he describes, that you, or anyone else the court appoints to be her financial guardian, would not be permitted, under the law, to provide any funds to your brother or for your brother's support. It would be different if your daughter was an adult rather than a minor, and she chose to provide for her uncle, but she is not, and her financial guardian cannot give away her money without getting into serious legal trouble.
Tell him that you understand that he is uncomfortable with trusts, but if the estate is put into a trust, for the benefit of you, your brother and your daughter as beneficiaries (with your daughter also being the successor beneficiary) then that will allow you to provide for your brother out of the estate. Tell him that he can explain to his lawyer exactly what he wants to happen and the trust can be written to do exactly what he wants to happen. If he doesn't want the farm sold, then he can include that information in the trust.
Also, suggest that he verify the information with his attorney, so that he will know that you are telling the truth about being unable to provide for your brother from the estate, if its all left to your daughter.