If my ethnic identity is grandchild of my grandpa Wolf Buchsbaum then my life is defined by the decisions he made, and he lived in a different world. I will never be able to respond to my world as he responded to his world. If I demand of myself that I do so, then I will have a burden of guilt — an unpayable debt to the past.
I think a better way of thinking about it is that I am the grandpa of my as yet unborn grandchildren. That way my current decisions decide who is in my ethnic group and who isn’t and what that group means to me.
So an Indonesian person might turn out to be in my in-group because he or she might also be a grandparent of my future grandchildren, or a great-grandparent of my future great-grandchildren.
Of course that also puts that Indonesian person in the family of my grandpa, Wolf Buchsbaum.
It also gives me the role of linking my descendants to my ancestors, or points out that I have that role.
What is true of my flesh-and-blood children is also true of my mind-children: my thoughts and other creations. I do not tell if my thoughts are true by tracing their ancestry — pointing out how they come from reason or sense data. I tell if my thoughts are true by looking at their descendants: seeing if they lead to a fruitful interaction with the world.