I am assuming here that your Dad has passed away and I am sorry for your loss.  You can challenge - or contest - a Will and the validity under various theories.  Undue influence - which would be improper persuasion or a misuse of a relationship to take advantage of a person's weakness - is one of them and that which I think applies here.  You are going to have to prove that your Father did not make the Will when he was in his "right mind" or "sound mind" and that he lacked "testamentary capacity" at the time that it was executed.  Seek legal help here with this matter.  Good luck.