Can an Amish woman whose husband has died marry to another man again?

Yes. According to Bible interpretation of most conservative Anabaptist denominations (which include Amish, Mennonites, and Brethren), a woman or man may re-marry if their spouce is dead. However, if they got a divorce, they would not be allowed until the ex-spouce died becuase that would be considered the sin adultry.