There are specific legal standards that you must meet in order to demonstrate that a will was made as a result of "undue influence" or "incapacity." Those are the terms courts generally use, as opposed to "duress." These are distinct issues, but from your brief case description I can't determine which, if either, apply to your case.
Generally you have three months from the time the will is admitted for probate to file a contest. This is only meant as general information as I am not familiar with the particulars of your case. I would suggest that you contact an attorney immediately to determine if you have a case and when you must bring it.
Best of luck to you.