Pj042893:Uncle suddenly fell ill at age 73. Was diagnosed and had about 10 days before death.
Fell ill with what? Many illnesses have no effect whatsoever on brain function. If you want to make a claim of incompetence, you have to prove his mind was so affected that he lacked the ability to execute a will. The standard for competence is state law, and you didn't mention the state. But in general, the standard for competence is very low. Typically all that is required is knowing who your immediate heirs are, what property you have, and that you are executing a document to dispose of your property at death. In order to prove incompetence, you'd likely need expert medical testimony as to his mental state when he executed the will. It is not an easy challenge to make.
Pj042893:Brother was isolating him and pressuring him to write and sign a will.
That raises a possible issue of undue influence. It is not "duress." Duress in legal terms means some form of illegal pressure is applied to make him execute the will, e.g. putting a gun to his head and threatening to pull the trigger if he didn't sign. But the details of the pressure applied are extremely important. It is not a problem for folks to encourage others to leave them stuff in a wil. It happens all the time. It only becomes a problem when the pressure applied is so great that it overcomes the testator's own will and leads the testator to make a will he otherwise would not have made. Again, this is not an easy challenge to make.
You'll have to consult a probate attorney in the state where your uncle resided to see if you might have a decent shot at a will challenge and what that will cost you.