"This birthright of Aster must be shared, my sons. Only by using your amulets together to bring back the togetherness of our family will you be able to form the key to claim what is now yours."
This statement finally makes sense to me.
Both of the Aster Twins' amulets must be brought together by the graveside of their father, Enton Aster. I have had many troubles searching for these amulets, but it is clear to me now that they were buried with the Aster Twins long ago.
The only way for me to obtain the inheritance is to find the graves of Bodan and Heric Aster to retrieve each brother's amulet.