How can I fight this and have the court admit my dad’s will?

The process is that the Will is admitted and he files a challenge to it. The burden of proof is on him. You have to check with a local probate attorney, or the one you already used [why do you not want to use him again because he costs too much?], but normally the judge will have a conference in which a trial date is set up as to the Will. Your brother will have to present a Dr. to testify your father was not of sound mind when the Will was signed [if it was all in his handwriting, no witnesses are required but it sounds like it was not and the notary stamp and signature would prevent it from being a handwritten Will] as there is a presumption he was and a handwriting expert that that is not his signature. You should present as witnesses the notary, your husband, and yourself [also his wife if you know she will co-operate but why would she if she got nothing] to testify all of you saw him sign and he was lucid at the time and appeared to know what he was doing, and a declaration from the last physician that he saw saying he was of sound mind at that time. Whether you are a good or bad person has nothing to do with the Will contest as it is not relevant as to whether he signed and knew what he was signing. It does not really go to the issue of undue influence, but the judge might let it in so your brother can not object he did not get his day in court. You should be able to send out normal discovery questions to find out what evidence he plans to present. ?It sounds as though he has a very weak case. The question becomes whether you feel comfortable enough to handle the trial your self, which I suspect you do not.

The judge will have to decide. I'm bothered by your statement that only the notary and your husband were present. Unless the will was entirely hand-written by dad it would require two independent witnesses. You should not go to court without an experienced probate lawyer.

Your brother has to prove his claims. Have the notary there to discuss the will formation and signing. Wife to support your efforts in following your dad's last wishes.