Bert is a German ending for a surname
Not only. There are French and British names that end in ..bert, and probably others too. Seibert however is a German name.
Not that everyone with a German name is German.
So it still should count as German.
That doesn't make much sense.
You don't say what country you're in. If however some of your family were immigrants to wherever, you should be able to find some record of the move. Also, the death certificate of the family member you're interested in should say where they were born.