The Will that was written for your dad is NOT valid.Your dad has to be mentally competent to write his own Will or be able to dictate the terms of the Will. Your dad has to sign the Will in the presence of two disinterested witnesses, who also sign.  Failure to comply with these requirements invalidates your dad's Will and are grounds for contesting the Will.  Another ground for contesting the Will is undue influence.If your dad is still alive and is competent to write or dictate another Will, the new Will should state at the outset that this Will revokes all prior Wills.If your dad has passed away, file a petition with the probate court contesting the Will that was written by your stepmother and signed by your stepsister on the grounds discussed above.If your dad doesn't have a valid Will, the rules of intestate succession determine inheritance.  Intestate means dying without a Will.  Under intestate succession, your dad's surviving spouse inherits his entire estate and the children don't inherit anything. If there isn't a surviving spouse, the estate is divided equally among the surviving children and if any deceased children had children (your dad's granchildren) had children, those grandchildren inherit the share their deceased parent would have inherited had their deceased parent survived.