The party who can bring a suit is the party who has been damaged. As of now, only your father's estate has been damaged, so the executor or executrix could perhaps bring the suit. Otherwise, depending on the date of death of your father, it might be an action that you can bring on your own - as a person who should have recovered under the estate. This will have to be answered by a local TX practitioner.
I can tell you that if your claims are true, then you have the ability to initiate an action against your brother for economic duress and fraud (forgery).
The elements of duress are (1) a threat or action taken without legal justification; (2) the threat or action was of such a character as to destroy the other partys free agency; (3) the threat or action overcame the opposing partys free will and caused the party to do that which he or she would not otherwise have done and that which he or she was not legally bound to do; (4) the restraint was imminent; and (5) the opposing party had no present means of protection. Chapman Children=s Trust v. Porter & Hedges, L.L.P., 32 S.W. 3d 429, 443 (Tex. App.CHouston [14th Dist.] 2000, pet. denied). Economic duress may be claimed only when the party against whom it is claimed was responsible for claimants financial distress. Deer Creek Ltd v. North Amer. Mortg. Co., 792 S.W.2d 198, 203 (Tex. App.CDallas 1990, no writ).
The elements of fraud are: (1) a material misrepresentation was made; (2) the representation
was false; (3) when the representation was made the speaker knew it was false or made it recklessly without any knowledge of the truth and as a positive assertion; (4) the speaker made the representation intending that the other party act upon it; (5) the party acted in reliance on the representation; and (6) the party thereby suffered injury. Formosa Plastics Corp. USA v. Presidio Eng=rs & Contractors, Inc., 960 S.W.2d 41, 47 (Tex. 1998).
Note that it is a crime for a notary to attest to a document that was not signed by the party who is representing to have signed the document. Your attorney will help bring this document into evidence properly.
Finally, your father's will itself will provide a great deal of help to a judge. The will shows your father's intentions. When we get in front of a judge about a person's estate - the judge will always look to see what the person's intentions were when they drafted their will.
Best of luck. Please accept and feel free to follow-up.
M. Strisik, Esq.